Chaos Emeralds

Video games have Bonus Stages that shower the hero with points and lives, but some games go further, and place optional Mac Guffins deep into their bonus stages. The reward for the player that hunts them all down is often a Golden Ending or other reward, but the game can be completed without them.

Usually, a bonus stage can be replayed if the Chaos Emerald in it was missed the first time, but since there are limited opportunities to play the bonus stage, collecting all of the Chaos Emeralds is never a guarantee.

Most popular with side-scrollers on the 16-bit era, particularly on the Sega Genesis/Mega Drive which had battery save on far fewer games.

Sub Trope of Gotta Catch Them All, which is a Sub Trope of Plot Coupon.

Trope Namer

 * Sonic the Hedgehog has six Chaos Emeralds, each with its own special stage, which could be accessed by having at least 50 rings at the end of certain levels. If an emerald is missed, the special stage can be replayed after playing the others, but there are only 10 chances total. Collecting them all slightly changes the ending. The 8-bit version doesn't have Chaos Emeralds in the special stages; they're hidden in the regular stages instead.
 * Sonic 2 has seven emeralds. This time, special stages can be accessed through any checkpoint, leading to the potential for a lot of special stage attempts, but in practice it's tricky outside of the first and fourth zone. Aside from changing the ending, this game let you turn into Super Sonic with 50 rings, which meant high speed and invincibility at the cost of rings. The 8-bit version of this game also had the Chaos Emeralds hidden in the regular stages instead.
 * Sonic And Knuckles had a limited number of big rings hidden throughout the game which each gave one chance at an emerald. In addition to Super abilities for Sonic & Knuckles, a final, secret stage could be unlocked if the emeralds were collected as Sonic. However, as mentioned later, locking Sonic 3 onto the cartridge would allow for battery saves, and infinite shots at the challenging special stages.
 * Later Sonic games tied the collection of chaos emeralds to the plot, turning them into straight MacGuffins, with a few exceptions.

Side Scrollers

 * Ristar has a bonus stage in every level, each with its own treasure. There's only one chance to get each treasure (and only if the bonus stage for that level is found). More treasures reveals more secret codes at the end of the game.
 * Cool Spot has UNCOLA letters. You need a certain number of spots (depending on the difficulty) to play the bonus round after each level, and hopefully get a letter. The best outcome is to collect and keep them all, but they can be turned in for a continue each.
 * Dynamite Headdy has seven unmissable (though skippable) opportunities for an intermission bonus stage. It needs to be cleared four times (after which it stops appearing) to get a combination number (which is re-randomized every time you start a new game) for use at the end of the game, which allows you to fight a Bonus Boss if entered correctly.

Turn Based Strategy

 * In Fire Emblem: the Sword of Seals, the Chaos Emeralds were found in the form of the eight legendary weapons. The eight legendary weapons must be obtained from completing gaiden chapters, bonus levels that could only be accessed if the previous normal chapter had all its hidden objectives fulfilled (such as completing the level before an X amount of turns have passed, keeping specific units alive, ETC) upon completion. To progress to the final arc of the campaign and gain access to the Good Ending, you must obtain all eight of these weapons AND keep them(and a freebie Sword of Plot Advancement) intact (they must have at least 1 charge left) before completing Chapter 22.

Action Adventure

 * Prince of Persia: Warrior Within had optional, hidden life upgrades spread across the game. Finding all of them was necessary to obtain the Water Sword which lead to the game's True Ending.

Side Scrollers

 * Donkey Kong Country 2 and 3 have unusual examples. You get bonus coins for completing the bonus stages, and these are required to get into the Lost World.
 * Donkey Kong Country Returns repeated this with Puzzle Pieces, which are also found outside of the bonus games, and they unlock some concept art and a music test.
 * Sonic 3 (and by extension Sonic 3 & Knuckles). Unlike the other old Sonic games, this one had a battery save, so getting all the emeralds is a matter of time and persistence.
 * Most new Sonic games use the chaos emeralds as straight-up Mac Guffins. Sonic Heroes is a recent exception where they follow this trope. There are keys in cages in various... interesting locations throughout the stages. A Power character breaks the cage, and once the cage was broken the key just had to be picked up like a ring or some such. Losing your rings meant losing the key (due to this, people usually use Team Rose), and getting to the end of the stage with the key got you into the special stage after it added up your score. You could only get the Chaos Emerald if you got the key in the second stage of any theme (basically, second act) - the first act just got you points and lives. Getting all the Emeralds and completing the story for every team unlocked the Last Story (True Final Boss and the true ending).
 * In Sonic Colors, it's actually impossible to get the Emeralds the first time through, due to the complicated procedure required to get them. First, you have to start collecting lots of special rings, of which there are five in each Act. You can start getting Emeralds once you have at least 120 of them; you need all of them to get all the Emeralds. Then, you have to finish the Sonic Simulator stages unlocked by the special rings; completing all 3 in a group (for example, 1-1, 1-2, and 1-3) unlocks one Emerald. The reason it's impossible to get all the Emeralds the first time through is because some of the special rings in earlier Acts can only be reached by using Wisps unlocked later on. Getting all the Chaos Emeralds doesn't affect the story, but it does unlock the ability to play as Super Sonic; however, this disables Wisp powers (sometimes tweaking level layouts correspondingly), so it's more of a Bragging Rights Reward.
 * 6 hidden items in I Wanna Be the Guy serve this purpose, as collecting them all just unlocks the Boss Rush. However, only one of them can be Lost Forever, being stuffed away in the Metroid section.

Platformers (3D)

 * The Tawna bonus rounds in the first Crash Bandicoot let you save your game if you got to the end, and two special bonus rounds had a key which each open a secret level.
 * Crash Bandicoot 2 had hidden or otherwise hard-to-get gems that would open a new ending once all of them were collected.

Other

 * Pickory features various "secret items" to collect, most of which are hidden in optional secret levels. They do not currently unlock anything, so it's just for the extra challenge.