I have a collection of side-projects, all combined are making in the ballpark of $500/month:

https://mathlegame.com/ mostly via ads) - made this one when wordle got popular (

https://reversle.net/ (mostly via ads) - made a few weeks after mathle

https://slashdreamer.com/ (subscription) - wanted to do something with stable diffusion - this was the most useful one for me (since I am using Notion a lot)

Working on https://ogtester.com/ now.