We've covered how to make a beer battery before, but if you've yet to get into the home brewing scene, you can make a smaller battery with a copper rod and a bit of water or vinegar.
Besides the rod and your liquid of choice, you'll also need a bit of aluminium foil, some wire, and a small cylindrical container in which to hold it all. As far as tools go, you'll just need some basics: scissors, pliers, and a glue gun. It's a fairly easy set-up to put together, and what's even better is that you can recharge it after it dies just by refilling the water. It won't power anything too gargantuan, but if you're sick of buying AAs all the time, it's a nice, rechargeable solution for a small calculator or something similar. Hit the link for the full instructions.
Water Powered Calculator [Instructables]