Close to Denver:
Rampart - often crowded, as mentioned. If you can, go with someone who knows their way around. I ride there a lot - 2-3x/week, because it is close. I typically start early, try to be on the bike at 7am, and I only really worry on weekends on the way back to the parking area - the closer you get to Rampart Range Road, the scarier it is. The trails are all ATV width, whooped near RRR, and bermed (except the new trails). IE, you can go pretty fast there, if that's your thing - but LOTS OF BLIND CORNERS.
Weekday mornings are "safe." The farther you get from RRR, the less congested it is. There's ~110 miles of trail there; the "posers" hang on roughly 20 of those miles. I like Rampart for what it is - but I'd not go there if it were not so close.
Woodland Park/717/Divide trails - you'll hear all three names, they're all the same thing. Rampart revisited, but 20? years ago. Similar topography, less traffic. More jeep road, but better _trails_ - they're not as motocrossed out. Just as scary as Rampart on a weekend morning near the Divide parking area. Everything is "717," hence the name. 717A, B, C, etc. D is very good. Last time I was there, I wound up on a closed trail three times. Got talked to by a very nice ranger. Two of them did not feel illegal at all - one was simply an intersection in the woods, turned right, looked like fun. Wound up dropping down a ridgeline, right up to the backside of a "no motorized vehicles" sign. Oops. The other one, I took a trail right next to a trail sign, wound up coming out in a campsite, where the ranger was waiting for the campers to put out their fire (and give them a ticket). She was nice about it, reminded me that if it is not clearly marked as a motorized route, it is CLOSED, no matter what it looks like. I went back and looked at the sign I passed - yeah, I see what they did. They were trying to say "the trail goes this way and that way" - but not the way I went. Fair enough; she could have given me a ticket easily. Careful at Woodland - there are a few trails which certainly LOOK legit which are not. That's FAR less of a problem than Rampart.
Farther out - Bocco Mountain, near Wolcott, about 2 hours west of Denver on 70. High desert riding, some STEEP hills, some very nice singletrack. Not super difficult from a technical standpoint, but the steepness is challenging.
Near CSprings - beyond Woodland Park, there's Captain Jack's - heavy use, no ATV (yay, I like narrow trails just as much as ATV people), somewhat small, lots of fun.
Near Boulder - not much, unfortunately. Left Hand Canyon, a rocky, small OHV area. ROCKY. Some singletrack there, but tilted toward "advanced." Some pretty sketchy stuff there. Fun, though.
Above Boulder, near Nederland - Caribou Flats, Rollins Pass area, behind Central City - all jeep roads, gorgeous views.
There's a ton of riding - email me - ircmannix -at- gmail dotcom - if you want more specific directions.
Are you looking for views? Singletrack? Fast jeep roads? Forest? Desert?
We have it all. I believe you need a nonresident OHV permit (you're _likely_ to get checked at any of the above places, save Wolcott). Spark arrestor, too.
Lots of places to ride - hope this reply is not too late, but someone might find it useful.
Iain