The LudStuff blog has a nice essay with photos on making tabletop rivers.
modeling
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.
Making A Spiked Barrier – Palisade
Intructables has an article on making a “spiked barrier for a Rohan gaming table,” but this sort of palisade would be great for a huge variety of periods and scenarios.
What Terrain Do You Need For Wargames?
I’ve been in this hobby for forty plus years, so I forget what it is like to start out. I’ve got a lifetime’s worth of wargames terrain to fall back on. But if you’re just starting out, here’s a primer on wargames terrain that you may need to start.