I am surprised I hadn't thought of Algonquian and Great falls parks. My house was about 3 miles from algonquian on RT 7. Time changes many things. I moved out when I was 12 and up to that point I never knew fat tires existed. It was kinda funny looking back, since my old nieghborhood is really good on paved non-sidewalkish trails. I was amazed how tight and challenging they still were, even after 7 yrs of XC racing.
The W&OD was another of my favs. Rail trails are always so effective for seeing scenery and getting somewhere. Probbably once this snow goes away (still 7 inches here) I will be coming down for a ride trip.