Scitoys has a great tutorial on how to build a working hydrogen fuel cell with objects from around the house. And the best part is, the whole thing will take you 10 minutes or so, which makes this a perfect project to do with your attention-span-lacking kids to introduce them to renewable energy concepts. Especially if there's a science fair or something coming up.
Plus, it's a really great DIY illustration of how the cell works, if you're curious yourself. And who doesn't want a hydrogen fuel cell next to their fridge?