Well, I made a working Hyperion Battle Cruiser once, complete with Laser cannons and Yamato Cannon.
I'm currently making a Marza Dreadnought, complete with Missile canons, auto canons and Planetary Devastation canons.
I'm also working on a massive underwater city, as well as a Fortress based on top of two mountains, with a bridge over the top connecting each side [Think Ostagar from DA:O].
I also built a rather large Dwarven City by hand, and poored in lava and all sorts of other stuff.
Then there's the floating island and city on top that I built.
Hopefully some of these will give you ideas. Rule #1 for anything though IMO is to include some redstone contraptions somewhere that do something fun, interesting or fitting for your construction. Planning not only the construction, but where the redstone is going to go so its out of sight but still does its job is far more challenging that just building something, especially when you're linking up 24 dispencers, 3 TNT cannons, lights and several pistons to a central control panel in a room above the rest of the construction. Try hiding all that [Its actually hilariously easy, but it takes a lot more work].