Chip'n'Dale hold 3 keys. Here is how you unlock them and what you get from it.
Key Area Reward
1 Post Office Cheats
2 Print Shop Pictures
3 Mickey's Garage New car
To get keys 1 and 2, find 9 acorns. To get the
final key, beat the chipmunks best lap times.
Cheats And Puzzles
Cheats are hard to obtain. To unlock them, beat
Pete in a racing world. Then, get 9 acorns and
visit Chip'n'Dale(their treehouse is north of
Casey Jr.). You can also use 20 Disney Dollars as
well. When you pay 9 acorns or 20 Disney Dollars
on Secret Challenge 1, he'll give a key. Open the
Post Office, then choose the Check Mailbox option
once you beat Pete with one character(make sure
it's the same character that beat Pete). You
MIGHT get a letter with 2 cheats. Examples are
like Disco Mode(which makes funky colors light up
a race track) to Submarine Mode(which lets you
pass through objects). Also, when you beat Pete,
he'll give you an item. At the Post Office, use
it to ship Pluto to a racing world.
Get more cash by taking Pluto to dig on the
ground, looking for dropped Disney Dollar Bills
on the ground, and competing in races muiltiple
Win first place in a character race and defeat Pete's henchman to unlock their picture.