Data nerds will love this one: EPA has this really cool calculator where you can enter energy data (such as gallons of gasoline or kilowatt hours of electricity) and it will calculate what that means in terms of emissions, such as car miles driven in a year. Combined with a Kill A Watt Meter, math lovers could spend hours calculating things like how much they could save by retiring the TV in the guest room (with the bonus of fewer mother-in-law visits).