N.B. I'm terrible at this so far, but I also run into this problem frequently, and my best idea is to keep an ~evergreen list of "things I can do quickly that are useful".
E.g. short bit of exercise/stretching/yoga, quick walk, read 10 pages of a book, make a healthy meal, deal with a couple of tedious admin-type emails, etc. I don't think most "real work" tasks fit into blocks that short for me (they may for some personality types/types of work), but there certainly are other useful things to do with that time.