Closest to Denver and a not too terribly challenging hike is Elk Falls, it's really pretty and it'll leave you feeling accomplished at about 10 miles. The Flat Iron hikes near Boulder are much shorter but harder hikes, although the views are stellar. And if you don't mind driving farther, then I'd head toward Vail or Aspen and do one of their trails to see the excellent leaves. If you're not acclimated to the altitude though, you might want to stick to some of the shorter and easy hikes if you go that route. But the drive is almost worth it on its own especially if you take Independence Pass which should still be open in October. And, of course, if you're up to it then you can't go wrong doing one of the 14ers! That would depend on if they get any snow, but typically early October there are a couple options near Leadville for those.
