[Question] Do you have algorithms for passing time productively with only your own mind?

Things that require nothing, just you. At least in the short term. (e.g., it’s ok to think about what question would be worthwhile to ask on Lesswrong.) I myself sometimes have specific questions from my studies and/​or practical matters, which are excellent for this purpose. But the supply of these questions usually is much less than the time I have for pondering them.