I invented mental puzzles to keep me entertained during class, including a stint in my teens where I worked out the base 12 multiplication tables.

Anyone else willing to share some specifics or can you expand some more on this? I'm still not past compulsory lectures, and I'd like to know how people keep their mind sharp secretly inside their skulls when everything moves so slowly you'd want to put time on fast forward. I usually review Anki cards, but that's not always possible.

What kinds of puzzles would be generally useful not just to programmers and other math oriented people? Mental arithmetic to a point is certainly useful for almost anyone.