Horsetooth Mountain Open Space: Horsetooth Falls - Fort Collins

Location: see website for different trailheads
Why Go There? In/out or loop hike includes a scenic waterfall, and options for short or longer, easier or more strenuous hiking just a few miles from downtown Fort Collins. Very popular, expect to share trail; open space park entrance fee (which helps support maintenance of the open space).
More Information: Horsetooth Falls and Rock Trail Loop + Horsetooth Mountain

Poudre River Trail Corridor - Greeley, Windsor

Location: Eastern most trailhead: Island Grove Regional Park, Greeley; western terminus River Bluffs Open Space Trailhead, Windsor (see website map)
Why Go There? 25 miles of paved, shared use, maintained level trail with eight easy access trailheads (bathrooms at a few of them), offers short strolls to longer level hikes with wild life, agriculture views; popular with bicyclists, skateboarders, runners, walkers; seasonal special events include an annual full moon walk (June), full moon bicycle ride (September), fundraiser Party for the Poudre day (September).
More Information: Maps

Echo Lake Trail - Idaho Springs (closest town)

Road to Mt Evans Summit open Memorial Day-September pending weather
Location: Park at historic looking Echo Lake Lodge (restaurant, gift shop/snacks), reachable on Highway 103. Restaurant open daily during season.
Why Go There? Level short distance trail circles Echo Lake; 10,600 ft elevation; connects to several other trails: Chicago Lakes, Bear Track Lakes trails, which offers access to Mount Evans Wilderness Area, many other hiking trails, and Mt. Evans summit; when aspens turn, fall colors can be spectacular.
More Information: Echo Lake + Mt. Evans Summit Hike

Additional Resources

Visit Estes Park - Estes Park
Gateway to Rocky Mountain National Park's eastern entrance  has several scenic walks in town, surrounding areas

National Park Service - Rocky Mountain National Park
Information, trails (300+ miles of trails)

Rocky Mountain Hiking Trails - Rocky Mountain National Park
A commercial website (has ads) for Rocky Mountain National Park's recommended hikes by category

Our Gardens - Fort Collins
Gardens on Spring Creek small 18 acre city owned botanic garden open daily to stroll; schedules activities; 2145 Centre Avenue

High Plains Environmental Center - Loveland
Bird walks, demonstration gardens