What makes you believe that what reedlaw described is not to be considered mental downtime? I have made quite the opposite experience: when consciously reflecting and deciding about my mental downtime in advance, it turns out to be the most recreative. It's worth noting that I made bad experiences with: tomorrow from 7-730 I will be reading/running/cooking/medidate/watching an educational video for recreational purposes. The way to go for me is to predefine a set of activities which I support based on a principle. When in the situation for donwtime, I then choose from this set.
Edit: typo