Productive downtime. Another one I'm still working on, but is based around the idea of pursuing tasks that are enjoyable but are still beneficial, as opposed to time wasters like watching tv, playing computer games etc.

Another approach is to try to increase the quality of your downtime rather than increase its productivity. For example, do deep breathing meditation. Laughter has been shown to restore willpower depletion, and I've found watching cartoons like The Simpsons to be much more rejuvenating than watching stressful live-action TV shows.