In the spirit of High Fidelity:
Top 5 things I like about bike riding:
1. Riding old bikes that I and mostly my LBS keep tuned up. I'm amazed that a 40-year bike can ride so well. I wouldn't depend on a 40-year old ... anything else.
2. Used to be some hills I had to sometimes walk the bike up. Either they've gotten flatter or I've gotten in better shape.
3. Contact with the streets, the bike trails - as other have commented, seeing the city from a point of view invisible from cars.
4. Exploring and getting into places I've never been.
5. Biking is is the only exercise I've ever done that, when it's time to stop, I want to keep going.