I like ruined buildings on a battlefield. Most real battles were fought over valuable landmarks such as towns, bridges, hills, etc...and ruined buildings lend themselves to representing that in gameplay.
We play ruins as rough terrain that provides cover. Because area terrain that provides cover is pretty powerful, I like to make ruins smaller than forests to make it difficult to fit a large number of models inside of it. I like ruins to be 6x6 or smaller. The ruins we are building today are 6x6.
A ruined building with attached walls can make gameplay difficult at times, so we are using cork corners to suggest the walls of the building, but they can easily be moved.
Step 1: Tear cork into shape to make corners. I make 2 corners per ruin template.
Step 2: Glue the corners together with tacky glue.
Step 3: Smear mod podge onto your ruin template.
Step 4: Gather stuff to put onto the template. This is a bit tough to see against the plywood, but I have kitty litter, pieces of cork, and pieces of foam. You want the end product to turn out kinda flat, so don't use any really large pieces here. Sprinkle the stuff onto the template. You could also add planks, I beams, sheet metal, or other cool bits here.
Step 5: After it has dried, coat the whole thing with Mod Podge.
Step 6: Paint the templates with a medium brown color.
Step 7: Looks like my camera minion forgot some more steps... We drybrushed everything with some light greys, and then painted the cork and the foam white. We added our flocking to the edges and any large patches of empty space on the template.
The finished piece of terrain, on a battlefield.
And the corners come off to simplify movement.