Painted the boards bestial brown. I needed a lot of brown, and cheap, so I painted a large piece of cardboard in bestial brown, then went down to Bunnings (hardware store) and they color matched it in a tub of acrylic paint for me. I ended up giving it two coats, waiting 2 hours between, as it said on the paint tub. (first photo is only after one coat)
My battlefield in a box river set fits along the length ways of the board, falling short by a half inch at either end, but that's okay.
I only had a small bit of static grass left (more on order) so I tried out the splotchy grass pattern idea, with thin random runs of brown. The brown will have grit glued to it, and washed devlan mud, like I did with the roads.