Well the what is certainly open for more discussion as is I guess the how.
With what in mind getting a base line of system non specific terrain is the best idea, boring old walls, ruins emplacements and the like. finding a way to make nice trees would be a winner as well. Everyone needs the basics and most of them can be made to suit a range of scales.
That yellow mould they had was quite interesting, it looked like a mould for making moulds =D For the how, I reckon a winner would be using the expanded foam like what your affrica table houses are made of. I am pretty sure topmark have it or something like it.
RFoam100 Rigid Polyfoamhttp://www.topmark.co.nz/articlelive/articles/24/1/Urethane-Foam/Full-View.html
U-F-RFOAM100-VT Urethane-Foam Rigid 100 500gm $ 42.00
U-F-RFOAM100-T Urethane-Foam Rigid100 1kg $ 63.00
U-F-RFOAM100-S Urethane-Foam Rigid100 2kg $ 105.00
Mostly as it reduces postage costs (always an atractive thing) plus it is tack free in and ready to demound in about 5 minutes.
It expands 8-10 times it size and certainly looks the part. What I dont know though is how much presure it will excert on a flexable mould and if it would cause deformation because of said pressure. Which could likely be sorted with thicker moulds or more ridgid RTV which means more cost or shallower detail respectively