I've got a couple of things I'd like to try.

Firstly I'd like to take a crack at whole house audio. I'd look at using something like Shairport and a set of amplified speakers in each room, but also look at running something like Softsqueeze as well to cover all bases. I can use either the built in ethernet or a WiFi USB dongle to make installation straightforward, instead of running line level audio or speaker cabling through the house. I could even look at using the built-in GPIO interfacing to automatically power on and off the amplifier as and when it's being used. The cost of the R-Pi makes it the cheapest part of the whole exercise.

Other than that, I suppose I'll look at building my long awaited Empeg MK3. My Parrot Asteroid has been a serious disappointment. Great hardware and form-factor, let down by really crappy player software.