Back before I had a driver’s license and a semblance of a life, I actually used to mod games a lot. As in, I created my own mods and played with them. Halo for the PC was extremely fun to mod, and it was often easier to just make your own than it was to deal with the patching system that was commonly used to create mods. Also: Exception errors.
Modding is how I learned how to use 3DS Max, I had a couple of partially completed custom maps done before I lost a hard drive. Lately all I’ve played with is metadata editing though.