#1. Glacier Gorge
In my opinion this is the most beautiful spot in Rocky Mountain National Park. Spectacular scenery, beautiful wildflowers, cascading waterfalls, and beautiful alpine lakes are what you will find here.
#2. Blue Lake - Indian Peaks Wilderness
A wonderful day hike up to Blue Lake in the Indian Peaks Wilderness Area. This short hike takes you up to a beautiful alpine lake surrounded by stunning alpine peaks.
#3. Bergen Peak
A beautiful hike up to the summit of Bergen Peak (9,708 feet). Expect to see wildlife, wildflowers along the trail, and excellent views from atop the summit. This is an excellent hike and a must-do.
#4. Boulder County: Longs Peak
Located in north-central Colorado in the Front Range, about 70 miles northwest of Denver, and 10 miles southwest of Estes Park. The most famous Colorado climb, this alpine peak requires a long, strenuous hike.
#5. Canyon Loop Trail
If Grandma��s House were at trail��s end, this would be a fairy-tale ��through the woods�� experience. And nary a big bad wolf in sight�� The Canyon Loop Trail weaves through the expansive Betasso Preserve.
#6. Arapaho Pass Trail to Lake Dorothy
This is a fantastic day hike up to Lake Dorothy via the Arapaho Pass Trail, one of the best trails in the wilderness area for viewing wildflowers.
#7. Lake County: Mount Elbert
Located in central Colorado in the Sawatch Range, about 11 miles southwest of Leadville, and 4 miles northwest of Twin Lakes Reservoir. The highest point in Colorado, this Fourteener provides a popular hiking spot.
#8. Chasm Lake
Chasm Lake lies nestled in a rocky alpine cirque below the towering summits of Longs Peak (14,255 feet) and Mount Meeker (13,911 feet), the two highest summits in Rocky Mountain National Park.
#9. Bear Peak
A strenuous but great hike up the summit of Bear Peak (8,461feet), one of Boulder��s most visible landmarks. The Bear Peak Trail follows Bear Creek up beautiful Bear Canyon, then makes a steep ascent.
#10. Maroon Bells-Snowmass Wilderness Trails
Nothing but superlatives roll off the tongue when contemplating the Maroon Bells�CSnowmass Wilderness: the beautiful symmetry of the Maroon Bells, trails leading over nine passes above 12,000 feet.
Jeffco Open Space has quite a few that fit your criteria, some that come to mind are White Ranch (especially upper White Ranch), Alderfer/Three Sisters, and Mathews/Winter
http://www.co.jefferson.co.us/openspace/
Head up I-70 to Clear Creek County for Chicago Lakes and Hell_s Hole trails up Mt. Evans, Herman Gulch, Beaverbrook Watershed and Loch Lomond /Mt Caroline.
There are some beautiful state parks that are fairly close: Castlewood Canyon, Roxborough, Eldorado Canyon, and Goldengate Canyon. They do have a daily charge, but a yearly park pass is worth the investment if you do a lot of hiking.
http://parks.state.co.us/
http://www.nps.gov/romo
Otherwise there are great hikes all over the place depending on where in Colorado you start from.