[Marc P.]

I'll second (or third) the carpet. Anything involving liquid or something potentially messy can cause all sorts of trouble in terms of electrical systems, not to mention a mess to clean up when removing robots from the field. Water, snow, oil, ice + 120 amps of power = recipe for disaster. Concrete would be interesting, but not readily avaliable as a surface in all venues.

I also like the variety idea. Perhaps one section of the field could be lexan, another shag carpet, and yet another something like astroturf. The most practical materials are the ones which can easily be carted around from regional to regional, applied over any existing surface, and are as re-usable as possible.

Barring those conditions, I'd say rolls of sod would make an interesting field. Nothing beats a lush green lawn for robots to play on!