Architecture

Creating 3D versions of buildings can be fun. Especially if you can get your hands on the
actual architects CAD (Computer Aided Design) files.
Sometimes you are just given blue-prints and got to measure by hand the drawing
according to the architects scale. 
Doing architecture walkthroughs is where your observational skills is most needed.
Because you will most likely be asked to make the animation look as real as possible.
So keep notes of how light & shadows affects different surfaces. Scan in a lot of real textures
like brick walls, wood, stones,...
Its amazing how much your models will look when you have a real texture applied to them.

Also lighting your 3D scene is most crucial, cause no matter how detailed your model is or how
realistic the textures looks if the lighting is not right it will look bad.