A heavy cylinder, such as a roller, or something else which will compound the surface would work very well. Water and sugar to make it stick.
I would not use concrete and tarmac like for onroad, I think dirt and clay would be a better choice, and sculpture it well to have flowing jumps, bumps, and contours to drive over - plus it can be changed every so often if you have the diggers, manpower and compounders.
Have a look for some events in the USA, such as the Losi Race, Cactus Classic, CRCRC etc.. as these will give you some great ideas
Chris