I fly to & through DEN frequently. I agree with the others here. Because the airport is remote to start with and takes a long time to get between curb & gate, 6 hours is about the shortest layover I'd want before going downtown, and that's without a major convention taking over the city. With 6 hours you could go somewhere nearer to the airport grounds, but where? And to do what? Probably you'd end up cooling your heels at a restaurant. For that, I'd just stick at the airport and avoid the hassles of leaving & returning. DEN is far from the worst airport to have to kill a few hours at. Look around; there are a few halfway decent restaurants there. Visit all 3 concourses and see what they have. Browse some of the gift shops. Sit at one of the bars and knock back a few drinks. Bring your laptop and pay for Internet if necessary. Consider buying a day pass to one of the clubs if you don't already have a membership.