Scattered throughout Metroville are fifteen locked chests, each one can only be opened by a specific character and, when opened with that character, rewards you with a new toy to play with in the Toy Box.
Mr. Incredible Costume
Mrs. Incredible Costume
On building along south side of road going through middle of Downtown. Next to white pole you can knock over.
There is also a special vault that requires all five characters to open. As there are so many characters for the items to be spread around in their individual chests this Vault will only give you three items total:
Metroville Toy Box
After finding the set reward(s) the remaining chest(s) will give you a randomly chosen item from this list:
Monument of Heroes