1. The Great Lighthouse of Armok: Build a tower at a coastal site to the highest Z level, preferably out of red material. Decorate higher levels with gold, silver, platinum, and clear glass/rare gem windows and such. If magma is available, create a magma fall via pumps, and/or light a lignite bin via the magma and use it as the beacon.
2. The Hanging Gardens of Dwarfalon: Carve out a gigantic cavern on a map with underground water, but leave a multi tiered somewhat pyramidal structure. Smooth it but don't engrave. Create a waterfall to drop water onto the top level of said structure then drain it into a chasm or bottomless pit after it irrigates all of the tiers. Wait for tower caps to grow and presto, instant wonder of the underworld. Engrave the walls if desired, decorate with gold, and possibly make a lavishly decorated interior with all sorts of waterworks, engravings, and pretty statues.
3. The Vault: Carve out a several story high room in a mountain, preferably on a site with more than one mountain. Engrave the entire thing (use scaffolds to engrave higher walls) and then mine out ALL of the ore in your area and make it all into items, then place the items in the vault. Possibly make subsidiary rooms for weapons, armor, ect. Decorate the hell out of everything, make it as high quality as possible. Place all artifacts in the vault, if you have valuable wood clear cut it and make stuff with it and vault it, move all of your booze and food into the vault (lock it behind a door to make the next step go as quickly as possible) then when the map is exhausted, make tons of coffins and place them in a subsidiary room or understory to the vault, seal off your original fort with walls, or collapse it if you want, and lock all of your dwarves in the vault.