Depending on your personality,

composing a melody

or a piece of music can either be extremely frustrating or a relaxing way to pass the time. If you find you fit into the latter category then

ReBirth for the iPhone

or

iPod Touch

might be just the app for you. ReBirth gives you a virtual synthesizer to work with to

create your own music

from wherever you are. It's fully loaded and will take some experimentation to figure out all the options but for $6.99 it's a budding rock star's dream.