Not a CO resident, but I passed through Denver on an off-roading trip over Easter. If you're ok with driving a few hours, Grand Junction has tons of offroad trails and places to ride. I think it's about 4 hours from Denver. That whole area near Fruita/Redlands and the UT border has some pretty scenic trails and a cool history with dinosaur discoveries/excavations. WOuld definitely make a good weekend trip.
Not familiar enough with the Denver area to recommend anything closer though...