There are so many great and not-so-great ideas on the LifeHacker site including this one I found showing you how to use C cell batteries in place of a D cell compartment in an emergency situation:
There are some other interesting things on this site like creating make-shift air conditioning systems using cold well water (others have made emergency air conditioners using beverage coolers, fans and copper coils): DIY Heat Exchanger and Make Your Own Air Conditioner.
There is this one showing you how someone made hand washing more efficient while filling the tank of his toilet. [JWR Adds: I would recommend skipping this one. The implementation shown uses plywood which cannot be kept sanitary. It also might result in a smelly toilet tank if you use an non-chlorinated water source such as well water or spring water.]
And here’s one with a video demonstrating how one can cheaply acquire 8 – 1.5v button cell batteries from 1 – A23 12v battery:
Well, there’s enough on this LifeHacker site to keep you busy for some time. Enjoy!, – Tanker