A couple of years ago I made a set of LED firefly's for my girlfriend's valentines present. They were little free-form circuits built around an attiny and a couple of small, flexible solar panels. These worked great for a few weeks, but then the batteries ran down and I discovered that the panels weren't able to charge them up high enough to light the LED (That's what you get for building things at the last minute and not testing them!).
I've finally got around to redesigning these (hopefully working this time). I've gone for surface mount components on a small PCB which gives them a much tidier aesthetic. I've also added a boost converter so that I can eek a bit more usable juice out of the battery before it drops below the LED's forward voltage.
I'm still not convinced that the battery will charge, I've blindly thrown it in because it looks nice and is about the right size. I get the impression that NiMHs don't really do trickle charging so my naive approach of sticking the output of the solar cells directly onto it's terminals may not work. I'll probably experiment with swapping them out for a super capacitor if I can find one that can store enough charge and is small enough.