Not a "side project" in the usual sense, but I teach as an adjunct professor (mathematics) every so often, and at a rate of about $1000 - 1200 per credit hour (depending on the institution) I can make a little under a grand a month after taxes. Teaching can be fun for many people, and I think it keeps me grounded in the fundamentals, where my day job is entirely application oriented.

The trick is that after a few times around, the courses require a bit less preparation.

Adjuncting is an absolute nightmare if you are trying to do it as a full-time job. But as a side thing for beer money when there is a staffing need? It's pretty light on stress. Since I am not gunning for tenure track (or even a full time position), there are little to no stakes involved.

Weirdly, I think the folks in my position (teaching as an adjunct as a side gig) along with the tenured full-professors on the opposite end of the academic spectrum are actually very similar in that regard.