Thought you might be interested -- my friend Harvest and I implemented this adaptation of Disney's Touche touch sensing system for Arduino, and used it to turn a mug of water in to a musical instrument. The extra parts cost about $1-2; everything else (Arduino, breadboard, mug) you probably already have lying around.
And it was so much fun! We got pretty sensitive and robust gesture sensing working -- we could tell how many fingers they had on the outside of the mug, when they touched the water's surface, etc. So then we used ChucK (a musical programming language) to turn it into an instrument.
Full blog post, including github w/ all relevant code and sound files, as well as links to an Instructables:
Theoretically this should work with any conductive surface (e.g. people, plants, large metal objects, etc) but we haven't tested it on other things yet. Would love to hear from anyone else who wants to try this. We owe a lot to other people who have posted similar things; it's adapted from Disney's Touché technology (full credits in the Instructables post/github).