Rocky Mountain Arsenal Wildlife Refuge | always free
The free Rocky Mountain Arsenal Wildlife Refuge in Commerce City, Colorado (near Denver) is a beautiful and pristine National Wildlife Refuge, representing an impressive cleanup and conversion of a former weapons and rocket fuel processing facility. The views of the mountains from this perfect spot on the plains are quite impressive.
Nearly nine miles of gentle trails allow the visitor to view a variety of wildlife while taking in the scenery. Many species of wildlife call the refuge home, including Buffalo (Bison), Coyotes, and Bald Eagles.

Begin your visit by checking out the displays at the large visitor center and grab a map on your way out. My favorite short scenic walk is the Lake Mary Trail and floating boardwalk.
Hours:
April 1 - October 16: 6am to 6pm Tuesday - Sunday
October 17 - March 31: 7am - 5pm Tuesday - Sunday
Visitor Center is open 9am to 4pm.
Closed on Mondays and all Federal Holidays.

Directions:
Take I-70 to Quebec Street.
Go north on Quebec Street 2.8 miles to Prairie Parkway.
Turn right at Prairie Parkway and travel 0.6 miles to Gateway Road.
Turn left on Gateway Road.
Gateway Road will take you through the entrance and to the Visitor Center.
For the latest details check their site: 

Free Trips Random Panorama
Upcoming Colorado Free Days
-
Sat 05-25-13 - Mon 05-27-13:
Boulder Creek Festival -
Wed 05-29-13:
Golden Kayak Event -
Sat 06-01-13:
Super Cruise -
Sat 06-01-13:
Denver Art Museum