Theres always the good old horse or mule that can go off road where 4 wheelers can't. They can be expensive to own but if there is a horse rental place in your area that may be an alternative. I've not rented one but I know there is a place near where I live that does that, not sure what it would cost though.

I have thought about getting one or two of those "dead sleds" that roll up and can be carried with you. They are suppose to be fairly tough and will slide over most surfaces. I think Sportsmans Guide had them for around $30 last time I checked.