How much work is making a Minecraft server from scratch? What things need to be implemented? I saw on the GitHub that placing and destroying blocks was implemented, so I’m guessing it’s a lot of work

I’ll go for something more simple, but that you still can spend hours on.

Tetris. Jstris if you want a minimalistic version and Tetr.io if you want something more polished.

I also like to play minesweeper, minesweeper.online is the best one I’ve found. I especially like the no-guessing mode which makes the game a lot less frustrating.