So, I've started work on a massive mine underground. It's not something I plan to do all at once, and I certainly don't plan to ever consider it complete. But several of my friends have taken to calling it the Mines of Moria, since my descriptions of exactly what I'll be doing with it seem to bring that to mind for them. (Basically, I'm digging waaaay out to the sides, putting columns in to keep the place lit with torches, and then digging down a few levels at a time, taking out everything and extending the columns after I'm done. When I hit bedrock, I'll mine out what I can, and fill it with lava, and cover it with a layer of glass. Potentially, I'll end up with my undercity sticking out of the roof of it, too. I'll have to be careful if I do that, though. Too much risk of falling damage.
Also, I'm going to make a massive, dimly lit library with the sole purpose of having it spawn monsters for me to hunt. Creepers be damned, I want my library dungeon!
I have quite a few other projects, like my massive glowing pyramid and my smoke and fire factory (complete with warnings to watch out for action movie heroes), as well as a massive snowglobe, underwater complex, a couple of large parks, and the rest of my city, complete with a burning ring of fire and cacti around the outer edge to keep the nasties out.