Another great tool for terrain to be used instead of Sandbox for skinning is Fredo's Toposhaper.
A good start if you have not seen it is an older video by Daniel Tal on 3d basecamp. Daniel has produced a lot of SU teaching materials, much of it regarding terrain work. You can search his blogs, videos, and books. I can't find my favorite basecamp video right now.
Valiarchitects has some good tools if you are looking for plugins for roads, walls, and buildings. I also use Artisan and Vertex tools for this work.