Before I had the space required to build my own permanent layout I used to take-over the lounge room (moving the dining table and lounge chairs much to my wife's annoyance) setting up temporary tracks for the weekend or even for a night of slotting. You could say I had permanent track and in particular, track scenery envy - most of us have been there.
After a year or so and after I had built a small diorama piece (mainly for photography) I decided to get a bit more adventurous with size and complexity. The concept was to have some small pieces (1.5x0.75 metres) of track scenery that I could incorporate into my temporary layouts. The pieces needed to be small enough that I could slip them under my bed or store them in a cupboard, etc.
I purchased a piece of MDF (8-10mm thick so it didn't warp) and started configuring track pieces. The objective was to make a series of curves and straights that were interesting to drive through. Next came the introduction of a little track elevation, using flat slabs of poly-styrofoam (I got this for free from my local supermarket boxing waste bins - they use styrofoam for transporting fruit) and craft glue I built up the tracks elevation by approximately15mm at a time. You can use plaster sheeting if you can't get access to foam, using plaster sheeting is outlined in the below 'Creating Slot Car Scenery' how-to.
IMPORTANT NOTE: Don't be too ambitious with adding elevation remember that the scenery piece has to connect into your temporary layout at each end so it therefore needs to be a little flat at each end.
I'm going to move out of first person here and make this a little bit of a How-to. :)
You then need to use a serrated bread cutting knife to shape the slabs of foam creating even grades and flat surfaces for your track pieces to sit on. Use large irregular block pieces of foam to form hills and mounded features either side of the track, be sure to cut the foam pieces into approximate shapes that look natural. The below photo shows the foam hills I constructed prior to painting to give you a concept, basically you are only limited here by your creativity.
TIP: If you're have difficulty deciding what to construct, then look up some famous WFC rally tracks on the net for inspiration.
Once you have terraformed your track piece (and the slot car track pieces sit comfortable across it), be sure to test it well. Make sure the slots can actually run across it and you have good electrical connectivity. Using craft glue, attach your track pieces to the terraformed foam.
Once the glue is dry you can start to add detail to your track piece to make it look more realistic. This example is a desert rally piece so small and medium textured rocks, sand, dirt, etc. were used. Again using craft glue, attach these items to the foam. In the above and below photos you can see more regular pieces (they look like wood logs or lumber) that have been glued to the track as obstacles for the RAID vehicles. These are pieces of balsa wood cut to shape and fixed with glue to the track surface.
Once you are satisfied with the level of detail you have added to your piece be sure to test it by running a few slots across it to make sure it's not too difficult to traverse. The final stage is to apply paint to cover up all the pieces of terraformed foam and give the track a realistic appearance. Be sure to have several paint colours for this stage (or blend some shades of your own by adding white or black to a base colour).
In Step 3 of the above 'Creating Slot Car Scenery' how-to it details the process of layering your paints to create realism and how to accentuate feature detail by using the painting technique of 'dry brushing'.
In the below photo I introduced a small water feature to breakup the desert look the piece had, this was just craft glue (which dries clear) with some colour added. If you do something like this with craft glue be sure to add the glue in thin layers or it takes too long to dry, in fact if may never dry if applied too thickly. :)
The most important thing to remember is to have fun, enjoy.