Wind-Up Key

One good way to show that something is a toy or toy-themed is with a wind up key. Especially complements Toy Time, or alternatively, Clock Punk.

It can also serve as a Weaksauce Weakness if something needs to be wound up to keep functioning.

Examples of Wind-Up Key include:

Advertising

There's a commercial for Pristiq anti-depression medicine which uses wind-up toys as a metaphor for how one feels when one is in the grips of depression, and when one is not.

Anime and Manga

In Rozen Maiden, the living dolls come to life when someone winds them up using a key.
Chachamaru in Mahou Sensei Negima needs to be wound up every so often. It feels very good to her, so it often causes her embarrassment in the hands of an overenthusiastic winder.
Ifurita in El-Hazard: The Magnificent World qualifies, with all the expected sub-text (the keyhole is at the base of her spine). When fully wound, she uses the key as a magic wand with Kill Sat level of firepower.
Nano, from Nichijou, desperately wants hers removed so it won't be so blatantly obvious that she's... not exactly human. The Professor who built her thinks it's cute.
In Yui Ayumi's manga Tina the Wind Up Toy, the titular character Tina and most of the toys that live on Wind Up island has to be wind up in order to function, and won't be able to move otherwise. There is one toy who doesn't have a key, though. (other than Tina's best friend)

Film

A wind-up car appears in a quick gag on Robots.
Hellboy: Kroenen had to wind up his clockwork innards with a key.

Literature

Tik-tok in the Oz books. Interestingly, he has three keys that operate walking, talking and thinking for him.
Played for creepy in Varjak Paw, when the cat hero finds a bunch of very, very creepy cats... that turn out to be wind-up toys.
The children's book Wagstaffe the Wind-up Boy, about a boy who was converted into a clockwork Cyborg by an oddball surgeon after being run over by a lorry. There's a brief scene where he looks at his reflection in a shop window, noting that the folded-down key is a barely-noticeable lump under his jacket.
Discworld
- In the novel Thief of Time, the History Monk protagonists use an experimental device called a portable procrastinator to artificially increase the amount of time they can use. It's worn on the back and powered by manually winding up the key. This is absolutely vital for their continued movement once time breaks, but only for one character.
For completeness, one should mention the clockwork mouse Mr Clicky, which the rats in The Amazing Maurice and his Educated Rodents often use to set off traps. They carry several with them in their travels, having agreed to discontinue use of Mr Squeaky since the sound of him being crushed was quite distressing to all.

Puppet Shows

The Muppet Show once had a robotic duplicate of Kermit the Frog, which had a wind-up key in its back.
The titular castle in the classic preschool series Eureeka's Castle has a giant wind up key on the castle itself that the giant would wind occasionally; usually when a guest performer appeared.

Tabletop Games

Yu-Gi-Oh Card Game has an entire archetype based on wind-up toys. Naturally, the key is a spell card.

Video Games

Super Mario World: The Mecha Koopa enemies have wind-up keys on their back.
- Bob-ombs also have them too.
The first Mario Party had a "how many times can you wind up the toy in X seconds" minigame.
In Donkey Kong 64, robot Kremlings called Krobots have these, and the theme tune itself even has a music box type wind up effect at the start, as seen here.
Luigi's Mansion has the toy soldier bosses.
The Mario vs. Donkey Kong games feature wind-up Mini-Mario toys.
Maple Story: Various monsters in Ludibrium have them
Lots and lots of these appear in the Syberia duology, seeing how it's literally crawling with all kinds of clockwork contraptions. Most notably, the train you are traveling on needs to be wound up on each station.
The Clockwork from City of Heroes, which are created from scrap metal, but almost always have one of these on their backs. The winders are also a common low-level salvage drop.
The Keymasters in Dynamite Headdy. Heather's mission is to collect all the keys to reveal the Dark Demon's castle.
In EverQuest, the Secrets of Faydwer expansion introduced the clockwork robots created by gnomes which all have a wind up key on their backs.
Pokémon in Pokémon Rumble are all wind-up toys, so the key is used to wind them up before releasing them. One such key is even the Big Bad.

Visual Novels

There are robots in Da Capo that are wound up by a key. They do have alternative energy sources, though.

Western Animation

Feed the Kitty: Marc Anthony takes a key off a toy car and pretends to wind up Pussyfoot with it to fool his owner into thinking that the cat is a toy. He succeeds, and Pussyfoot then gets himself into the aforementioned toy car, minus key. Marc Anthony finds it cute, until Pussyfoot manages to get the toy car working and starts driving erratically, possibly because of the aforementioned removal of the key.
Kilgore from My Life as a Teenage Robot is a pitiful toy robot, powered by a watch spring and a wind-up key, who dreams of destroying mankind.
Mechanicles from Disney's Aladdin animated series is often seen winding up his Clock Punk insect-like robots. He even has one dedicated to winding up all others, so he only has to wind up that one.
In the Donald Duck cartoon Donald's Diary, Don's nightmare of being a henpecked husband culminates in a key growing on his back, making him into a robot doing chores for Daisy.
Yogi Bear once put a key on his back and pretended to be a giant wind-up toy grabbing picnic baskets. He is stopped by a wind-up Ranger Smith.
A common gag on Droopy cartoons is for his opponent to wind him up with a key to send him toddling away like a toy.
In Wakfu, Big Bad Nox must be fed everyday with wakfu through a giant key-shaped tube (which fits into a hole in his chest rather than his back, oddly enough.)
In a Dave the Barbarian episode, Candy is able to tell Dave apart from Mecha-Dave by pointing out the key on its back. Later, Mecha-Dave stops his battle with Dave to tell him to wind his key.

Real Life

At one point when the locomotive Blue Circle was based at the Bluebell Railway, it acquired a (fake) key.