I highly recomend an app called BtBx (beat box), it's a drum machine/ synth sequencer app, really fun and intuative if you like electonic music and such. It's really the first recreational app I've found to be really entertaining. I am a musitian though so this could be concidered a buyist opinion, in any case I really recommend it. I have pocket guitar and the novelty of it wore off pretty fast for me.
Oh, and if you end up enjoying BtBx, you could step ip to another app called BeatMaker, Which is way more elaborate but costs 20 bucks.
Adrian D