Everything listed below you have to do as a second player (Chu's brother).
On each level you have to find a hidden chewing gum.

level 1 - hit a hole with red eyes under the stairs of the repair shop
level 2 - hit a stick where the owl had sat after you saved it.
level 3 - hit the call board
level 4 - hit a pink metal lid and then a black pipe behind the bear (below the window).
level 5 - As a second player press "Use/Hit" (E for keyboard) in the Shop
level 6 - hit AC power surge and then push a box all the way from the right side of the screen to kick it into the window next to the power surge.
level 7 - hit a sphere outside the screen on the right (it's hidden behind a pile of rocks).
level 8 - hit a fish which jumps out of the water
level 9 - hit dying fish. You've got to be careful as it can run away (jump away) from you. But still you have a chance to hit the fish when it will start jumps away back from Chu towards you.

Everything listed below you have to do as a second player (Chu's brother)
On each level you have to find a hidden stash of mushrooms and gather all of them.

level 1 - hit a snow pile next to the black hole with red eyes
level 2 - hit a snow pile to the left of the shop's door
level 3 - hit the Deer-guy
level 4 - hit the white barrel with exclamation mark on it
level 5 - hit a big white rock
level 6 - hit the big trashcan next to the cauldron
level 7 - hit a bush next to the mummy's building (after the level's event with the mummy). You have to call the rain for that to happen (2nd task).
level 8 - hit the mailbox