This is a very mild violation, but I like to play these puzzles: www.chiark.greenend.org.uk/~sgtatham/puzzles/
…except that I create a custom difficulty level which is quite a step below the easiest difficulty and then I almost rather speedrun the puzzles.

The Rectangles puzzle at 5x5 size has been my crack for the past months and I’m at about 13 seconds now (using my phone as input).

I mean, it’s very casual speedrunning. No one cares about my time, so I actually never timed myself before just now. But yeah, I just like the different challenge of thinking fast rather than complex.