It seems to me that half the fun of this project is seeing the changes you're making to the waveform. With that in mind, I'm going to include a tiny oscilloscope:
It's the XMEGA Xprotolab. It will be mounted on the top of the enclosure, unless I'm able to mount it behind the lid and have the buttons still accessible. (Maybe I should find someone in industrial design to do the enclosure. They could probably design those plastic nubs that are set in the enclosure surface, which are used to press tactile switches...)
Unfortunatly, it costs about $50. I'll have to wait to design the enclosure until I have a bit more money and can order one of them.