A rest day should consist of a ride where you do nothing but light spinning. It is great aerobic fitness. You can also rest at any time on the bike by spinning and should be part of any ride after a big push. When I sprint, I follow-up immediately by spinning until everything comes down to a normal pace. It's like stretching on the bike in a way.

I know a lot of riders that start and finish every ride with stretching exercises. That helps prevent injury. Spinning also prevents injury when done right.