Flickr/James Paull 71 miles Cutting through Arapahoe, Denver and Douglas counties, the High Line Canal Trail, one of the nation's longest urban trails, includes everything from public art to ponds, forests, residential areas, golf courses and industrial zones. Bikers, horse riders, rollerbladers and joggers are often out and about, but considering that 350,000 residents live within a mile of the trail, many stretches still feel very remote. Find more information on highlinecanal.org. Distance varies While Red Rocks Park is best known for its world-famous amphitheater nestled between 300-foot-high rock formations, the 640-acre Denver mountain park also offers two hiking trails: the 1.4-mile Trading Post Trail and the six-mile Red Rocks Trail, which leads to Matthew Winters Park. During concert season, these trails are packed, and the roads to them can be clogged with traffic. In winter, they're considerably less crowded and offer the same stunning views of the plains and the rock formations.