Opening the Sandbox

The point in a video game, especially an RPG, where you're finally able to do all the sidequests, go anywhere on the map, and so on. Usually coincides with getting the Global Airship, in games that have that. May still be this even if you technically can't go everywhere; there are many games where the beginning of the Very Definitely Final Dungeon is also the Point of No Return.

Usually a very good time to get the Infinity Plus One Sword or go for One Hundred Percent Completion. If the entire RPG is like this, it could be a Quicksand Box. This point also comes shortly before the Point of No Return, or after if the game has certain forms of New Game Plus.

MMORPGs are a special case. Though most do not have a true 'end', they end up becoming more top heavy over time as new content additions target veterans who have seen and done most of the things in the game. This effect is particularly severe if several consecutive updates do not increase the Cap but give already capped players new options, and is the reason that established players tend to perceive more freedom in a game than newbies despite being more aware of its limitations.

These points are sometimes near the ending, so SPOILERS.

Action Adventure

 * The Legend of Zelda series tends to do this multiple times each game. In most games, after the introductory village and dungeon, most of the world map opens up, though in the first game the world could be explored right away (though getting a sword, which is in a cave on the first screen, is a good idea). And then, once you've completed about half of the dungeons, a large chunk of the map that was hidden or inaccessible is revealed, usually through some specialized game mechanic like Dual World Gameplay.
 * Ocarina of Time. After you (finally) leave the village, all of Hyrule Field is open to you. When you get the Master Sword, you then gain full access to most of the areas you visited previously (even if you're still missing some key pieces of gear.)
 * The Wind Waker: You can start sailing anywhere shortly before you start the Tower of the Gods. Most people wait until they get the Ballad of Gales, though, because otherwise you'd have to sail all that distance by yourself.
 * When you finally break the Skull Kid's curse in Majora's Mask. As a human, you finally have a decent melee weapon and can leave town.
 * Twilight Princess lets you out into Hyrule Field once you've freed Ordon Province from the Twilight. However, true to form, the game doesn't truly open the sandbox until you drive back all the Twilight and get the Master Sword.
 * Link to The Past lets you into Hyrule once you rescue Princess Zelda from the catacombs. Once you complete three dungeons and slay Agahnim, the entrance to Hyrule Castle becomes a gate to the Dark World (which holds eight more dungeons.)
 * Oracle of Ages opens up three times, each time when you gain a new method of time travel.
 * In Skyward Sword the sandbox is opened practically straight after you open all three portals, getting the Clawshot also opens up a couple more oppurtunities.
 * In Okami, you can go back and get most of the missing collectibles once you've bought the Double Jump and unlocked Kabegami's brush technique, allowing you to climb walls, but it's a lot less traveling to wait until you've gotten the Mist Warp technique so you can teleport between certain sacred mirrors. Most walkthroughs advise you to just wait until you right before the Point of No Return and start backtracking from there.

Platform Game

 * In contrast to its extremely linear predecessor, Super Mario Land 2 Six Golden Coins is far less linear. After finishing the tutorial level, the six zones containing the titular coins have no set order to do them in, and players are free to, for example, complete part of one zone and then work on another, leaving the rest of the previous one for later.
 * The Green Power Stars from Super Mario Galaxy 2. Justified since while half of those stars are very easy to find, the other half involves jumping off whatever planet you're on, and falling to your doom if you screw up because they're often placed out of reach for the player. It's best if you start with the easy ones first.
 * And before that, the remaining Prankster Comets.

Role Playing Game

 * Fallout 3 confines you to an underground vault, which serves as a tutorial level, in the early part of the game. You eventualy escape, gaining access to the rest of the world.
 * Final Fantasy I unusually has this at the halfway point (or if you're like most players, slightly before), when you get the Global Airship.
 * Final Fantasy VI has two: right at the end of the World of Balance and getting the airship in the World of Ruin.
 * Final Fantasy VII: Disc 3.
 * Also notable is the point on Disc 1 when you leave Midgar and see the World Map for the first time.
 * Final Fantasy VIII: Once the Missile Base, MD Level, killing NORG, and all that bullshit in Fisherman's Horizon are said and done with, you are free to pilot the Garden around. You can go everywhere except Esthar and the Bonus Dungeon in the ocean. In disc 3, you are given a proper airship and can truly go anywhere you please.
 * Final Fantasy IX: You can explore the ocean when you receive the Blue Narciss (ship) on Disc Two, then the whole world when you gain control of the Hilda Garde II (Global Airship). You upgrade your airship to the Invincible when you return from Terra at the start of Disc Four, but it does nothing that the Hilda Garde didn't.
 * However, this is all rendered moot if you keep up with Chocobo training. A Dark Blue chocobo can cross the oceans, and a Gold Chocobo can fly (so long as there's a forest to act as launch pad/landing pad). Makes Sequence Breaking easier, but you can't do much out of order.
 * Final Fantasy X: The game opens up a bit when you get to the Calm Lands, but really this happens when you get the airship permanently.
 * Final Fantasy XIII has No Sidepaths No Exploration No Freedom up to the point where Lightning's party reaches Pulse... in chapter 11 (out of 13). There's a living but utterly inaccessible cityscape around the straight path, which is the main reason most people accuse the game of being too linear.
 * In Secret of Mana and Seiken Densetsu 3 - After you get Flammie.
 * After beating down Dalton the second time in Chrono Trigger.
 * Most Pokémon games: About when you can use Surf and Fly outside of battle. If you can use all the HMs, you've definitely reached this point. The Gen IV games make you see (not catch, thankfully) all of the Pokémon in the Sinnoh Pokédex before opening everything up, though.
 * All core The Elder Scrolls games did this to you the moment the Tutorial Level ended, read, when you crawl out of the first dungeon. Morrowind went even further, throwing you out into the world right after your character build was finalized (so did Oblivion, but there was more gameplay during character creation there). The game even tells you: "You're on your own now."
 * In Baldurs Gate, this happened immediately after Gorion was killed. In the sequel, the moment you step out of Irenicus' dungeon.
 * Steambot Chronicles stops railroading you around when you finish the tournament. You could do some sidequests before, but were forced to remain in the same city or surrounding area until this point.
 * Mass Effect 1 hands you your ship and turns you loose after a couple hours of gameplay. Some people feel slightly intimidated by this. However, you still can't access all the star systems until you complete missions that unlock them, and there's no point at which you can freely go to any of the available planets; when you unlock the Very Definitely Final Dungeon, you lose the ability to go back to.
 * Similarly, Mass Effect 2 lets you loose once you finish Freedom's Progress and, but you can't explore all of the galaxy until after Horizon, about a third of the way through the game.
 * Mass Effect 3 opens up the sandbox after escaping from Mars, but more and more the star systems are gradually unlocked after each Priority mission.
 * In Knights of the Old Republic, once you get off Taris, you can go to whichever planet you want any time you want.
 * Dragon Age, post-Lothering.
 * Legend of Dragoon: There is a point near the end of the game where the party acquires a strange manta-ray creature called Coolan, who can fly them to any destination (although if you plan to go too far back through the game, have the relevant disc ready).
 * Shadow Hearts likes to open all the side quests when the Very Definitely Final Dungeon appears.
 * First game: After the Float rises. You can't go back to China, however.
 * Covenant: Pretty much anytime, really, but the sandbox truly opens once the Stone Circle activates.
 * From The New World: Once you step into the Gate.
 * Shin Megami Tensei: After the End
 * The point at which everything is available in Tales of Symphonia comes VERY late in the game. Specifically, you need to enter the final room but not use the warp that leads to the final boss. At that point you can leave and do all the quests.
 * Most Dragon Quest games once you obtain a ship. Starting with Dragon Quest III, flight is possible and getting the ability to fly opens up the world further. (However, Dragon Quest IX doesn't give the player the ability to fly until after completing the main story, so the ship is the main mode of travel until then.)
 * Most Ys games start out linearly, gradually opening up the rest of the world, and usually providing you with a warp item to quick-travel to previous areas. Usually, there's at least one Point of No Return just before or at the beginning of the Final Dungeon.
 * In Romancing Sa Ga, the world starts opening up once you finish your main character's Prologue; as you visit different places for the first time, they're marked on your map and become more easily accessable.

Shoot Em Up

 * The "Diplomatic Relations" mission in Escape Velocity: Override is necessary for unlocking most of the non-human, non-Voinian mission strings.

Third Person Shooter

 * The 360 game Crackdown is already a GTA-ish sandbox game to start with. But once you've beaten all three gangs and finished the end-game you can roam freely around the city with all your powerups available and the option of re-starting any of your previous missions in a mode that feels much more like Opening the Sandbox than just having finished the game and being able to run around. There is also an DLC that adds God Mode, which has an option that also effectively gives you an Opening the Sandbox mode.

Wide Open Sandbox

 * In the computer game Spore after passing the first four objective-based levels you are free to explore and colonize the universe as you see fit.