One way to generate a terrain heightmap is to generate a number of random hemisphere shaped hills with random radius on random coordinates and blend the them all together. By setting a min and max radius the final look can be tweaked to something you like. The result might look like the following:

Link to more detail about the algorithm (also the source for the image):
http://www.robot-frog.com/3d/hills/hill.html

Most games that contains big cites doesn’t fill the buildings with proper interiors because of the cost in time and performace to do so. Instead they only apply a texture on the buildings exterior and make interiors only for a very few buildings that are of special interest for the player. With this method you can fake the interior of buildings without the need of much extra work from the artist, programmer or designer. The result can give very living cities.

Link to the paper describing the interior mapping in detail:
http://interiormapping.oogst3d.net/