Between the hills and the cars, bicycling is the most exciting way to get around the city!
Keep your eyes open, follow the traffic laws... and always, always, always watch for people popping open their driver-side doors into your bike lane.
If you get tired of the hills, the Embarcadero offers a beautiful flat stretch from AT&T park to the Aquatic Park, and beyond that only a small hill to the Golden Gate Bridge.
Bike politics is fairly hot as well, the SFBC is an highly active organization and counts most local politicians - including Mayor Gavin Newsom - amongst its supporters. The Critical Mass ride is worth checking out as well if you're free on the last friday of the month.