100 steps is a coin and you can get 10 coins a day.
So far, I've seen you can spend 2 coins to get pieces of a puzzle in the Mii Plaza, so far? I have 3/15 of the Kirby pieces! (The other way to get pieces is to walk past other 3DS systems with StreetPass)
The other thing is the AR Card games, after playing enough of the games eventually you'll unlock the shop, spending game coins gives you extra games to play, apparently alternative free modes or harder variations.