I have completed (at least in its current form) my tower. Which is good because I need to take a break to study and prep for finals this week...
Anyway, if you go under my tower and into my "mineshaft" there is a lot to explore, namely a bunch of obsidian if you keep going straight and then look around once you leave my "outpost" (the big open room with a bench and smelter and numerous torches)...