Add some rolling hills...
Simple plaster method:
To make a hill, start by piling up scraps of wood into the shape of the hill. Place screening material over it, and staple down the edges with a staple gun. Bend and shape the hill with your fingers, until the desired shape is chosen. Mix plaster of Paris in an old bowl (small amounts at a time, as once it's dry, it's DRY!) and spread it on with a putty knife, over the screening. While you are doing this, cover nearby track and the floor with a dropcloth or newspaper. Once the plaster is dry (see the package for drying times), sand it smooth to the desired shape. Water may also be misted on to allow surface shaping, but don't add too much water. To make mountains, simply use taller supports under the screening, and don't smooth the plaster as much- real mountains aren't smooth. Just glop it on there!
Make a frame, as above, using cardboard instead of screening. That is, build wooden supports and make a lattice or web of 1" wide pieces of cardboard over it. Staple the pieces down on the corners. If it's a small hill, crumple up newspaper and use masking tape instead of cardboard and wood. Mix the plaster to be thin, such as pancake batter would be. Dip sheets of paper towels in it, and drape them over the forms. As this part gets messy, be sure to cover the tracks and floor. You can also buy plaster-impregnated gauze, which you simple cut into sheets, dip in water, and apply. Then, putty-knife on a layer of plaster as above, let it dry, and paint it with a latex tan colored paint.
Glueshell is very much like hardshell, except that it is lighter and more flexible. The basic idea is to use ripped cloth instead of paper towels and thinned white glue instead of plaster. Dip the sheets in the glue and drape these over your hill structure. If you're building a modular layout, glueshell is for you. Although it's more expensive, the glue is pliable and lets the structure flex as you move your modules.