Currently, I have two main projects planned:
A giant stone cauldron that will fill with water from the river that runs through my map, that will flood the space between my fortress walls and the perimeter walls. The water will drain by being pumped back into the cauldron.
A big wall with a solid rock bottom level (besides secret hallways for my soldiers to move through) and a floored second level. Every screens-width or so (like 30 tiles) I'll make a tower. Like the great wall of china.
I'll probably do the wall thing first, since that will be simplest to build, and my marksdwarves can use it. I'll probably make a small scale water flooding chamber later, since I haven't worked with water very much and I don't want to deal with flooding my entire map (I have walled off about 8 screen's worth of tiles, and I'm at 125x50 tiles, that's a lot of water to pump around).
So for now, I'll probably stick with making a huge maze of traps for goblins to go through: first a couple cage traps (so I can make a colliseum), then a series of lever operated spike traps, then a bridge trap that leads down to whatever I feel like (perhaps a bunch of war dogs), then a series of atom smashers, all placed randomly, then a drowning chamber (in case I don't want my soldiers to get tired
) then if I let them get passed all that they can rush my marksdwarves in the towers.