i know this is kinda trekkie geeky but i think a tricorder app like from startrek would be fun. i dont know what it would do exactly but it would be interesting.
would be kinda funny if the app made the sound faster and faster based on the tilt of the phone... so you could move it closer to somebody (and tip it up slightly as you move) and it would seem like it was "detecting" them (or something about them).