Toy Story Midway Mania is filled with lots of toys that are very familiar to kids as well as adults that remember them from their youth. Part of the charm of Toy Story Midway Mania is guests see many toys they grew up playing with and it instantly makes a connection with the guest. Not all of the toys, however, in Toy Story Midway Mania are there purely for nostalgic reasons. In fact, some play a role in telling the backstory of the attraction including a toy that's seen both in and around the attraction: the Monkeys in a barel.
The monkeys are a key aspect of the storyline of the attraction because in the backstory, Andy has just received the Midway Mania playset. Once Andy leaves, the toys "come to life" and start playing with the midway games themselves. Because there are so many toys that Andy has now, the toys have even begun to spill out of Andy's room and out of the Toy Story film and into (and subsequently out of) the Pixar Studios itself. You see, the exteriior of the building is themed to be a scale replica of the real Pixar Studios and the Pixar building represents where films like Toy Story "live".
The first toys out of the building are of course the Green Army Men. The Green Army men use the barrel of monkeys to create a bridge across the Pixar Place ground and onto the other building. The monkeys serve as an important asset for the Green Army Men to complete their "mission". While out and about, they aren't all business like the Green Army Men and they explore and swing around the area. You'll find monkeys all over Pixar Place and Toy Story Midway Mania and some have even started some trouble.