More ideas:
Long ago I made a bunch of "bad going" areas for DBA by cutting out irregular shapes of flat material, gluing on bushes and rocks, then filling in the areas between with lots of dirt and a little grass. I used sheet metal as the flat material, but you could use plastic, cardstock, foam, cloth, etc., anything you can cut that lays flat.
Similarly, you could make a pond (irregular shape painted mottled, muddy brown/black, then varnished high-gloss clear), and make a bunch of mud-and-grass blobs to place on top of it. Use it as a pond sometimes, a swamp other times.
I also occasionally use my corduroy farm fields as bad going. In wet weather the mud in a tilled field can be much like soup.
Whatever you do, make sure the miniatures can still stand on it without toppling. My favorite thing to do is make a blob of cloth to mark the area of the terrain, then decorate it with removable 3D pieces (trees, buildings, rocks, lichen, logs, etc.).
- Ix