The enthusiastic developers of Parranda were at WWDC to show off their app, a simple one-man-band party in your hand. Parranda (iTunes link) has a lot of polish, too, with instruments that aren't just one big button, like some "soundboard" music apps. The cowbell, for example, allows you to tap on different parts of the instrument, which any decent cowbell player will tell you, creates very different sounds. Mostly. You can zoom in or out on the instruments, something you will have to see in the video, but it is a nice effect and adds to the experience.
Parranda has pre-set rhythms to choose from, plus vocalizations to add a little spice to the performance. Already a hit in Puerto Rico, I think Parranda has a good shot everywhere, as it's a fun little app. Perhaps a bit more than you're used to paying at $2.99, but I think it's a fair price for the quality of the app.