You can unlock more things in the game by getting rare coins.
If you get the rare coin in the first level "Po's Dream", then you will unlock four characters in Multiplayer mode.
They are: Tai Lung, A Woof Sister, The fat guy with the wrecking ball thing that looks purple and has horns, and one of Tai Lung's minions with the swords.
After you do the first panda stumble, walk back to the bottom of the hill that you started stumbling on, and you will see a green rare coin hovering above a box!