Hmmm...I never even saw this thread...don't know how I missed it.
Anyhow, here are some cool things...
If he has access to old cars, it would be cool to have those out there (like two parked side by side) that we could drive over...Hondas would work nicely.
Some logs place along the trail (straight across as well as canted) would be good.
A rock garden would rock (pun intended)!
And, of course, almost everyone likes a good mudhole. It doesn't have to be super deep...but I think at least 2 feet would be nice.