Living in Denver usually means driving to a trailhead. Many folks keep to a rule of thumb that it's a kind of mountain biking sin to spend more time in the car than on the ride, so if you're going to a trailhead that's an hour away you'd better ride for at least two.
More often than not, riders seem to gravitate to Buffalo Creek for all-day rides. Rightfully so; that trail system is one of the best around and it keeps getting better. However, in the middle of summer it can be almost as hot there as it is in the city. If you've had it with climbing up shadeless trails in deep sand, consider driving just a little further down Highway 285 for an entirely different kind of Colorado mountain biking. On paper, this twenty-two-mile ride with 3400 feet of climbing may seem like a mellow day out. Yeah, and on paper it should've taken Homer a month to sail home from the Trojan war, not ten years.
You can park in the lot that runs along 285, and you'll start climbing up the Colorado Trail heading west. The aspen forest here is pretty anytime, but offensively pretty in late September. Enjoy a fairly short and pleasant rooty climb. This is followed by a ripping descent down an exposed hillside into South Park (friendly faces everywhere, humble folks without temptation).
Once you see this dumb scenery, get ready for a little descending. Don't think about how you'll have to climb back up this hill when you're tired.
The trail climbs for the next several miles through quintessential Rocky Mountain forest. There are roots, rocks, logs, and stumps. It's not a grueling climb, but there are no uninteresting moments. It's gorgeous, moderately strenuous, and ten to fifteen degrees cooler than Buffalo Creek. Idyllic is a word that comes to mind.
Once you ride above tree line you're near the top. The trail forks before you summit Georgia Pass, and the sirens will call you to the nearby mountain pass. Listen to them. It's worth the extra five minutes to ride to the top and stare into the abyss. Just tie yourself to the mast so you aren't tempted over the edge. (By the way, it's best to begin this ride fairly early, or the afternoon storms very well may blow you off to Ilium).
To descend Jefferson is to battle the six-headed Scylla. It is to be drawn into the whirlpool Charybdis. You shall be swept away, cry out, flail your arms, and be lost in lengthy mortal struggle (all in the best possible ways, of course, though there may be bloodshed). The word "epic" used to mean something.
It's rocky, rooty, and wet, and it doesn't let up for a long, long time. There aren't insane technical features you'll have to walk, but cumulatively this trail is moderately difficult. It's relentless. It's awesome. It may remind some of Monarch Pass or Mountain Lion.
Crossing the narrow sea is sometimes easy.
At the bottom of Jefferson, you'll find yourself in a campground. There may be a direct way to meet up with the Kenosha Pass trail, but riding through the campsite at the end--while unmarked--will suffice. You'll be satisfied and probably ready for a beer, but your Odyssey is not yet over. While there are many ups and downs to go, only one true monster remains ahead. It's a long, exposed climb, but there's a rippin' descent afterward back to the parking lot.