For me, it's all about the joy of riding a bike. I finally learned how to ride a bike this year. Every bike ride has been a blast, even the ones in which I got bruised due to crashes and whatnot - part of the learning process I guess. I don't know if I can fully separate that from the joy of not being in a car - although now when I'm in a car, I feel like I'm in some kind of a cocoon that isolates me from the rest of the world a lot more than a bicycle does.
Favorite part is probably riding past the Jefferson and Washington Monuments, and through the National Mall. Riding along the Potomac River is pretty sweet too in the morning, when there's less traffic on Mt. Vernon Trail.