The yarn idea was one of those "opps I forgot a box of terrain" improvizations. The other cheap and easy way to go is cut shapes of various colors of felt. Dark Brown is a hill, light brown is a depression, dark green is a forrest, light green is a marsh, etc.
Or you can just spend 25 plus years collecting terrain like I have.
Crossfire is all about the terrain. I think it does the one thing that real combat teaches a soldier, and that our tabletop games often forget, the world is not flat.