Here’s a great tutorial in making some very realistic concertina wire for your 25mm – 28mm miniature games.
Modeling
Making Terrain From Cardboard
Although this video is focused on Warhammer 40K, I think the techniques shown for building terrain entirely out of cardboard are worth studying.
Making Tabletop Rivers
The LudStuff blog has a nice essay with photos on making tabletop rivers.
Poker Chip Terrain Bases
Carmen’s Fun Painty Time blog has a couple nice examples of terrain bases made from poker chips. He’s a very talented guy.