The last few years, the most fun I've had doing anything active, has been the sport of cyclocross. I must not be the only one, as cyclocross is one of the fastest growing sports in the US. Not only has the national sport of Belgium been welcomed here (we have had the last two World Championships in Louisville, KY) but we have been producing some great bikers.
Think redline the whole way. The 400 meter sprint of biking. Fast as you can for about an hour, or 30 minutes for newbies. There are skills involved, on and off the bike over barriers, is surprisingly technical.
It's spectator friendly. Most of the time you can watch most of the race from one spot. Spectators are encouraged to be loud and a part of the scene. It's laid back. It's the coolest of all the racers. People are there to have fun, not make fun of your lack of fitness, under priced crank or baggie shorts.
Cyclocross Is The Most Beautiful Thing In The World from ted burns on Vimeo.