One of the most popular trails in Boulder County open space system, the Mesa Trail runs about 8 miles along the foot of the Flatirons from Chautauqua Park south. Perhaps its prettiest offshoot is the Big Bluestem Trail, which creates a 4.2-mile balloon loop from the trailhead at the south end, just east of Eldorado Springs. With cactus and yucca near the start, pine trees in the middle and South Boulder Creek to dip your feet in at the end, it offers all you could ask for in a front range day hike.
http://co.boulder.us/openspace
 

The Burro trail, is one of several good day hikes in Golden Gate Canyon State Park west of Golden, is a roughly 5-mile loop through varied terrain with a short spur to a viewpoint at the top of Windy Peak. It’s rated as difficult, but hikers-only sections make it enticing. An easier option is the Horseshoe trail, also a hikers-only route, a 3.6-mile-out-and-back up a draw and through strands of aspen into Frazer Meadow, with its historic ranch buildings and backcountry campsites.
http://parks.state.co.us