Definitely echo recommendation of
Portals of Phereon, which is an honest-to-Bob actual (and difficult) game rather than fluf between jacking/jilling content. Actually more of the former than latter, for that matter. Hugely underrated considering the wealth of its mechanics, doesn't hurt that Syvaron is actually a pretty good artist with unique style as well.
Free Cities with the Mod That Shall Not Be Named™ if you liked the proto-management elements in this. Also highly interesting from the perspective of AI tools' impact on games, considering it supports StableDiffusion now. The future is here, watch it emerge in real time, heh.
Java 101 and Basic Good Programming Practices courses if you got your entertainment just from looking at the source code.