For me it was minesweeper clones. I got frustrated one day and decided to learn how to be good at minesweeper. After beating the medium and large boards a couple times I looked on Steam for minesweeper versions, and turns out there’s a whole genre of clones. Some of them are direct clones of the game, while others are very heavily inspired by minesweeper. The two best I played were Hexcells and Tametsi. Hexcells is stylish and is only hexagons (as opposed to the minesweeper squares), while Tametsi has squares, rectangles, and hexagons and is a lot more barebones. However I found Tametsi to be much harder. There were some levels on there that took me an entire day, and I think there’s like 500 levels.
After many years I’ve gone back to Assassin’s Creed games. Got some of the classic ones on sale and whoops…several hours gone again.
On the topic of minesweepers, you’ve probably already seen Hexceed on Steam. That one has gobble up a chunk of my time in the past as well.