Townscaper is probably the most interesting procedure generation "game" I played. The buliding are generated for an irregular grid, and yet everything fit together organically. It was always a mystery to me how it actually works. This article explained some of the mechanisms.

A game where you place planets in a system. The goal is to place as many planets as possible while avoiding collision or flinging them out.

It's an interesting playground for learning concepts like why small planets are likely located in near star orbit, three-body problem, etc.