A nice and simple thing to build in a city is a pyramid. I, personally, believe that a pyramid built out of planks looks the best, but you could make one out of any block you desire.
The main reason why i find this a fun build, especially for servers, is because although it takes quite a long time, it's an ideal candidate for a team project and can then be modified to act as a multi purpose building.
For example, as you can see i have turned this pyramid into a passive mob farm. This can be done by simply lighting the interior. You should also make sure that the pyramid is built in a biome that allows for spawning . This one won't work due to it being in a snow biome,
Finally, i suggest lighting the interior with glowstone. This adds a nice effect to the pyramid and allows it to keep it's shape. Keep in mind however, that this will affect spawn rates.