I'm happy to have not tried Kittens and will stay away from it. UP wasn't too bad. It was a time sink until I played all the way through it, and then wasn't too bad. The murderous game for me was 2048. I had to block the site in /etc/hosts to break out of the game. More recently, duckduckgo has embedded it as an easter egg in their search screen, so it's harder to block that way.

I also waste time on chess puzzles, lichess.org/training , but that isn't so bad since it's easier to pull away, and I felt for a while like it was helping my game, and decided to count that as improving my mind, as contrasted with 2048 which is pure inanity.