I own a Prius, I bike a lot, and I kick in extra money for "green" electricity, but there is a LOT more I can do!
The thing about biking is you can decide to do some of your driving errands by bicycle. That decision is good for the environment, good for your physical well-being, and good for your social life. You can't lose!
Biking can be more time efficient than driving and working out. On a bike you are traveling AND working out. That kind of multitasking can actually save you time. I like to say I don't have time to take the car - I should bike instead.
I also installed a high-efficiency wood burning fireplace insert, and I've been partially heating the house with it. When you burn wood you're still releasing carbon, but you're releasing the carbon that the tree pulled from the atmosphere. So the net effect is zero, as long as another tree was planted. Burning fossil fuels takes carbon that was in the ground for thousands of years and releases it into the atmosphere. That's bad...
I'm also very interested in the potential of the voluntary carbon offset market, but without regulation of some kind I'm not sure it's worth much.