Last summer, Jeff and I did a thing called the Zip car 'Low Car Diet.' The idea is that you try not to drive your car for a month - instead you walk, bike, take public transit, or use zip car. To make it easier, they provide you with money on a metro card, a bike-sharing membership, and a zip car membership (at least that's how it was in DC). They have the program in Boston, Chicago, New York, London, Atlanta, Toronto, and a bunch of others.
If you're interested in trying it, go to this link to sign up. And make sure you mention my name - if you get picked, both of us will get extra zip car cash!
You can find more about the program here: http://www.zipcar.com/lowcardiet/