Dwarf Mode and Adventurer Mode complement one another marvelously. Don't overlook the game's Legends screen, which becomes available after your fortress crumbles. Search through the history of your dwarven civilization and learn about all the various attacks against your capital. Investigate the goblins that harassed you for a decade, learn the name of their leader, train yourself to legendary status and attack their capital, with the ultimate goal of dethroning their demonic leader.
As for your fortress, a mature fortress makes for an excellent and ambient tomb. As a previous poster suggested, build lots of deadfall traps, tunnels, secret passages, levers, pressure plates, magma and water pipes and pumps with switches to turn them on and off. Make a zelda-esque puzzle for your adventurer to navigate, wherein a false move floods the fortress with magma, ceasing all hopes of future exploration. Require the adventurer to spill water onto magma to make a bridge. Hide monsters and rig cages up to levers, creating exciting and death-defying experiences for the adventurer.
Then, upload the save file and give forumites directions to your old fortress site and challenge them to clear the labyrinth. You'll have to come up with a goal, of course, and don't forget that loose items (furniture, goods, armor, etc) get scattered when the fort is abandoned. Only buildings and placed furniture stay where they are, so you can't hide a bunch of treasure behind locked doors (it'll still find its way out). If you were awesome (if) you'd make a lever at the deepest terminus of the labyrinth, and whence pulled this lever would collapse hundreds of supports within the fortress and destroy it all, ending the deathtrap that has beckoned and claimed so many adventurers...