GarageBand lets you play instruments such as the piano, drums and guitar. There is also a function called "smart" instruments, which is the place to be if you are not so musically inclined!
Here is the first song I created with GarageBand on the iPad:
There is also a Mac version of GarageBand that is equally fun to play with! You can create a song with the pre-mixed loops, capture audio and piece it together, or learn how to play an instrument with the lessons on GarageBand!
Whatever you like about music, GarageBand pulls it together!