Originally Posted by Moistfly
I don't think rest days necessarly need to be spent doing nothing but it is important to reduce the intensity of your training enough to allow your muscles time to rebuild.
There are two things you can adjust on recovery days: intensity and volume. Intensity (level of effort) times volume (time) is the important relation for determing physiological stress. If you go long, go easy. If you go short, you can up the intensity somewhat.