You know those plastic trays used by supermarkets for meat, etc? The flat parts are useful for making those subtle variations in ground level that occur in real life. It scales around 1/2 to 1ft thick in HO.Just cut roughly to shape, glue down with white glue, then carve the edges with a craft knife. I use cheap latex or acrylic craft paint 9on sale) for the basic colour, then the usual ground cover.
I'm sure others have found additional uses for this free "craft material." ;-)
Have fun, Wolf K.