This is a good place to look for ideas.
-You could work up to puncturing the land of spoilers the fun way: with a magma piston. Then dig too deep on purpose.
-You could start ALL the wars! Humans and Goblins and Kobolds oh my! Iron Christmas every friggin' month!
-Make a farm. Not for mushrooms and turnips and lame crap, I mean like Old MacDonald's farm but with Rutherers and Trolls instead of cows and pigs. For bonus points, name your animal tamer Urist McDonald.
-Perform science. Remember, the difference between science and screwing around is writing it down.
-Create an immortality room. Even after you "abandon" your fort, that vampire will still be locked behind his non-deconstructable walls without a pickaxe, stuck with no clothes, no food, no booze, and nothing to do but admire his really friggin' nice chair and stuff. When you give up on your fort, take comfort in the fact that even without your sage guidance, the Dwarves totally would have been able to avoid the Your Fort Has Crumbled screen.
-Find a way to make everyone in your fort weird. Make them all vampires, make them all werenouns, I don't know. Make them all footless by making them stand in 1/7 magma near stairs or something.
-If your fort has one entrance, make a labyrinth. If your fort has more than one entrance, make several. Fill them with fun/creative traps... like pressure activated skeletal carp cages.
-Pick a Dwarf, and do the Most Interesting Dwarf In The World thing. Lock them in a series of rooms and force them to do a certain type of labour continuously until they master it. (start with siege operation and butchering, everything else will go Rusty in the time it takes you to train those at all) Then hand them a crossbow or an axe or something and watch the world's baddest badass beat the stuffing out of absolutely everything with his/her insane metalsmithing skills.
-Make an unnecessarily awesome defense system. I'm currently building an obsidian magma-killsat tower. Why? Because... magma killsat tower.
-Replace every single wall in your fortress with a window. Glass is pretty cheap to produce, right...?
-Hold a battle royale. Save up caged invaders from several different spheres, put all the caged douchemonkeys in a room, attach all of the cages to one lever, stand waaaaaay back, and pull the lever. Record a video. For bonus points, put all of them in a room adjacent to spoilerland first, seal it off, have someone puncture the circus, and let the infinite demons into the mix. Humans vs. Goblins. vs. the underground wildlife vs. the aboveground wildlife vs. the undead vs. the circus.