Grow 1 of each possible tree that can grow in your biome. Since each tree uses up around 25 squares, twenty fruit trees(something like that) alone will eat up ~1/4 of 1x1 embark. Have to wait and guard them, though!
Have all your citizens descend from two pairs of elves/goblins/humans. No dwarves past first 2 migration waves.
Sit in your fort, breeding, so long that the surrounding world all dies - then go out centuries; possibly build fortresses by other races (via modding). Picking right immortal citizens above will help to ensure they don't leave or die out while you're doing another long fort.
Tame all the animals to masterful that show up in the fort.
Embark on top of hundreds or thousands of megabeasts; seal them off and get them breeding till FPS runs low. Then abandon the fort, releasing them to wreak havoc (I guess?).
Might be more interesting in an alive world with just few, maybe just a pair of each breedable megabeast. This way, can name them with dfhack so you recognize them later on.
Release the HFS and give everyone there a name.
Ghost fortress. Amass a necropolis.
If doing 1x1, definitely build a tower - and conquer hfs, for more room. I think dfhack has a script to add more z-levels to sky - something like extend-sky?
Build and test Larix's
eternity clock (Might need adjustment to fit into 1x1)