This is an excellent topic and I was hoping to find a set of great ideas here (really love the http server example). I was thinking of the concept of Katas in martial arts. What would be the Katas for programming I wonder.
Some thoughts:
1) Build your own database. Not a SQL db. Just something that takes records on a cli and enables retrieval. Persist the data. Support better queries/
2) Maze solver.
3) Snake game
4) Helper/Solver for something like Wordle.
5) Chat server (but I like the HTTP server idea. Annoying thing is newer HTTP versions have gotten complicated).
I think there are twenty to fifty of these little things we have all done in our career, and either forgotten them or it felt too natural to register. Wish someone compiles a list of such projects some day. It would bring back joy in some of the coder-at-heart folks lives.