I recently (well not so recently, but I am pretending it was recent) I purchased GarageBand for iOS on the iPhone. It's a nice little app & seems to have a tonne of features (it certainly takes up a lot of gigabyte-age on the old iPhone HD).
I only had a chance to noodle with it a bit, but this is what I came up with.
Maybe this will give you all an idea of how easy and fast it is to come up with little ditties using this gem of an app :)















