I've got a very nice fortress humming along with 101 dwarves right now, but I'm running out of ideas about what to do with it. Some cool features that I've already built:
-Indoor plumbing that draws from a nearby stream and supplies fresh water to the dining room and hospital wells
-A zoo (with a black bear, a wolf, a peregrine falcon, a porcupine, and -- just for heck of it -- a caged cyclops)
-An "arena" for prisoner execution and military training
-A legendary dining room
-Outdoor sculpture garden (so tastefully arranged!)
-A "danger room" for training my military (they're all "Grand Masters" at this point)
-4x4 bedrooms for all dwarves
-Every inch of stone has been smoothed and engraved
-All entrances (both above ground and into the subterranean caverns) covered by a series of cage traps and weapons traps
-Opulent throne rooms, burial chambers, dining rooms, etc... for all the nobles.
Most of my profit comes from prepared food (a few large farm plots and a growing herd of sheep supplying most of the ingredients, supplemented by gathered wild plants and trade-acquired food items) but I've also got a pretty good set of stonecrafters and bonecrafters, and my metal industry is busy smelting galena into lead and silver, then forging the lead into crafts and the silver into weapons.
The point is: what next? I need a cool challenge that could end in lots of FUN.