Southwest of downtown Boulder, the 43,000 acre Boulder Open Space has 10 loops of varying difficulty open to hiking. The two main access point are the Chautauqua trailhead and the South Boulder trailhead.
The Chautauqua trailhead is located off Baseline Road near 9th Steet. It provides access to the following loops: 1) Chautauqua - Bluebell - Baird - Mesa Loop (easy 1.6 miles); 2) McClintock - Enchanted Mesa Loop (easy 2.1 miles); 3) Royal Arch (moderate 4.0 miles); 4) Gregory Canyon - Ranger - Greenman - Saddle Rock Loop (difficult 3.7 miles).
The South Boulder trailhead is located south of Boulder on Broadway/SR93 about 0.5 miles before Eldorado Springs Drive. It provides access to the popular Big Bluestem - South Boulder Creek loop (easy - 4 miles).
Overall, Boulder Open Space is the 53rd most popular hiking trail of all 72 hikes in Colorado.