[PWalk]

Absolutely Pie,

First I used foam insulation for most of the track. It's easy to work with, cuts real well to just about any shape you want and at only about 1 inch thick you can adjust the grade of hills very easily therefore making it easier for a Bit to get up.

For making "rocks" and "cliffs" I used to Crayola's Model Magic (cheap plug!!) It's easy to mold, sets up quickly, and best of all it doesn't crack when it dries. Also it's lightweight which is great for when your carting your course around like I have.
DO NOT USE PLAY DOH! It may be cheap but it cracks and breaks all the time.

For decorations you can use a variety of things depending on the setting you want the course to be set in. I use a variety of stuff from sand paper on the hills(great for traction where you need it)
Deer Moss for trees and shrubs, and cinnamon for dirt (best tasting dirt I've ever tried).

I can go on and on so I'll post the rest in the track section.

PWalk