Garage band is a great app, if you have an apple product. I think it was even free. Both DD6 and DD8 like to fiddle around with song arrangements (programming a drum and horn line in, and then recording themselves singing or rapping along, for instance), and I use it to play guitar and drum lines while I play piano along. It took some time to figure out, though, and you don't have complete freedom to make the different instruments do exactly what you want them to. I think if you have a piano or electric guitar you can hook up to the computer and record your music.
DD6 simply jots down songs in a composition book as well. She'll play them on the piano and then figure out how to write them as music. It's a great brain activity!
