This isn't really a circuit idea, but more of a peripheral. I'm using a tiny circuit to build a watch right now. I haven't figured out yet how I'm going to rig up a watch strap to it. I'm sure I'll buy a cheap strap and attach it to some plastic or just strip down a cheap Walmart watch and stick my circuit on that. Wouldn't it be cool though to have a watch strap with a pre-drilled platform for attaching tiny circuits to?
Along with this, a watch-sized LCD board would be handy too, but not really critical for my project. Just sayin'. Combined with the bluetooth, could pair with your phone and display all sorts of secondary information, incoming calls, etc. Throw in some vibrating motors to notify you of alerts. And on and on.