I think I've mentioned before that I've started doing yoga.  I stop for a break on my bike rides or do it at the end of the ride, depending on where the park is located on my ride that day.  I also try to do it at home on my rest days.  I have incorporated many of the stretches I learned from Dr. Dan for my back and about a dozen yoga moves into about a half hour routine.  It works up a good sweat.  I also have found that it feels good to do it bare foot in the grass. 

I've noticed some improvement.  I used to be able to get into the pose shown above as a kid, but can't quite get there these days.  I will know improvement when I can get into this one.  Perhaps even a headstand some day!!! 

Give it a try.  I highly recommend it.  Here's a website I've found to be quite instructive, and there are no nudes, dispite the "unusual" name.  http://fuckyeahyoga.tumblr.com.