There are a lot of possibilities, so I'll just list my favourite 2 done on our server.
The easier one is a set of cannons. We built a ship approximately 100 meters long. Along one side of the hull, I placed 4 hidden dispensers. Flipping a switch would uncover holes in the hull. Pressing a button would then launch a salvo of arrows from the ship.
Having the dispensers hidden had a nice property - as long as the shafts were closed, the cannons were completely invisible from the outside.
Probably the largest and most complex building we built was a stadium with a mob grinder.
We started by building a standard grinder - many levels of dark rooms with water and a hole in the middle. Mobs are spawned and then fall down to be killed at the lowest level by lava. Water then moved all the dropped items to a single place to be picked up.
Then a friend added some control mechanisms for the grinder (continuing in the nice tradition I started). Should a need for maintenance arise, the grinder could be turned off. The flow of lava was stopped. Using pistons, the (quite wide) vertical shaft the mobs were falling through was closed off before the bottom. Then that barrier was flooded by water, so the mobs falling in wouldn't get hurt while the grinder was turned off.
Reaching 110 meters in height, and sporting a big control room, this already was the tallest and most complex structure in our world. But then a friend had an idea.
A stadium built from obsidian was built on top of the grinder. A redirect switch was added, so instead of falling into lava, the mobs at the bottom fell into a watery elevator.
The elevator brought them to the top and lunged them into the obsidian stadium, where they could be killed for fun and profit. (Their flow was, of course, controlled by doors.)
And should they get out of hand or should we become tired, the solution was also simple: with a flick of a switch, the roof of the stadium opened, allowing the sunshine to burn 'em to death. (I also remember having a building with rows of armed dispensers to shoot the remaining creepers; that might have been a different building though, I'm not sure.)
All this may seem complex, but remember - there are many parts which could be built separately. The mob redirection/control, the grinder control, the automated stadium or shooting ranges could all very well stand on their own. And they wouldn't be doable without a lot of redstone, pistons, repeaters and thinking.
Oh, and last but not least; my personal favourites are hidden doors and
mob player traps. I once built a network of tunnels that connected all of the buildings and spots in our valley, but were quite intricate, contained dead ends, pitfalls, lava traps and the like. And since the entry/exit points were also hidden from view, it sometimes looked like you were just walking along a hill, suddenly sunk into the terrain and reappeared atop a giant tree on the other side of the valley. (I liked leading people through them safely. :) Also, I had some hidden plantages right under everyone's noses and it took a week until they found them by randomly digging - pneumatic stone doors FTW.
I hope you'll find some inspiration here. :)