Cities: Skylines is a very good city simulator with built-in mod support. However you do have to write the mods in C#, so it's not the most beginner friendly.
If you know your way around PowerPC assembly, you could check out my Star Fox Adventures mods. I've made a nice build system that injects C code into the game, and the engine is very versatile, so there's a ton of potential. More reverse engineering is needed to really take advantage of it, though.
You could also get an emulator with Lua support and mod old console games. That's always fun. Again, requires some technical know-how, but not as much as you might think.
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