This Piezo Drum Kit from Spikenzie Labs looks like it could provide days of fun. They provide interface software that allows you to output MIDI data from your Arduino that this kit is connected to.

Thanks for the tip Andy, send us some pictures when yours is built!

"The kit contains the electronic parts required to make a drum kit. This includes the circuit board, resistors, diodes and pins. You supply the Arduino and the material to make the actual drum pads. Below you will find the easy instruction on how to make traditional looking drum pads, but you could also stick the piezos (the part the sense the hits on the drum) to many different surfaces. Imagine, playing your desk, lamp and telephone !"