Golden Clear Creek Kayak festival (Wednesdays in June)
The "Golden Community Rodeo" is a weekly kayak competition in downtown Golden along Clear Creek at the Golden kayak park. You can sit or stand along the creek and watch the competition. It's pretty amazing what these kayakers can do, and definitely worth taking some time to watch. Can you think of a better thing to do on a warm spring Wednesday?
Competitors are arranged according to class. This year the classes are:
Juniors
Masters
Men: Sport, Intermediate, and Expert
Women: Sport, Intermediate, and Expert
City Park Jazz
Looking for free Denver music? City Park in downtown Denver has a free summer jazz concert schedule with top performers.
Jefferson County Open Space Parks (always free)
Jefferson County Open Space Parks are probably the best park value in Colorado. A tremendous variety of parks and activities, such as hiking, mountain biking, climbing, and geocaching, all free to anyone.
Here is a list of their parks, maps, details, etc... 
Denver Cinco De Mayo Festival (May 3-4, 2014)
One of the best ways to experience Denver's rich Hispanic heritage is to visit the annual free Denver Cinco De Mayo Festival, one of the best things to do in Denver on a May weekend.
Western Welcome Week, Littleton (Aug 8-17, 2014)
Western Welcome Week is a festival in Littleton Colorado that celebrates our western tradition of community cooperation. This is the premier event in the Denver area for clubs, charities, and other non-profit organizations. Instead of the typical endless line of vendor booths, you will find tents set up to gain your interest and participation in various non-profit organizations. Finding a new hobby or charitable cause to get involved with is one of the rewarding things to do at Western Welcome Week.
Snowmass Summer Concerts
On summer Thursdays and Fridays the laid back atmosphere of Snowmass Colorado comes alive with their free Snowmass Village Summer Concerts. The music, set up on a Snowmass ski run stage near the upper village, is loud and wild.
When:
Summer Thursdays (and sometimes Fridays) from 6:15 to 8:30pm.
Where:
Snowmass upper village.
Schedule:
The 2013 Snowmass Village Free Summer Concert schedule is as follows:
June 20: Freddy Jones
June 27: The Dunwells
Friday July 5: CJ Chenier
July 11: The Revivalists
July 18: Mountain Standard Time
July 25: J.Boog
August 1: Tab Benoit
August 8: Nigel Hall
August 15: Robert Walters
More Info from the Aspen Snowmass web site:
Two Ponds National Wildlife Refuge, Arvada (always free)
Two Ponds is a very small National Wildlife Refuge which was created as a place for migrating birds to stop and rest.
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Monarch Mountain Free Day (April)
For 2013, Monarch Mountain is offering a free lift ticket in April if you purchase a $10 GEMS card.
Get the details and order your GEMS card here:
Location:
Monarch Mountain is located in South Central Colorado near Salida. Map
Learn more about Monarch Mountain Ski and Snowboard Area at their web site:
Ruby Hill Rail Yard
For about 6 weeks each year in January and February, Ruby Hill Park in Denver features a free ski and snowboard park known as Ruby Hill Rail Yard.
Longmont Studio Tour (September 29-30)
Aspen Nordic Center (always free)
For vacationers to Aspen, the Aspen Nordic center provides probably the ultimate cross country ski and snowshoe experience in Colorado, and it is entirely free!
U.S. Air Force Academy Visitor Center and Chapel, Colorado Springs (always free)
The United States Air Force Academy at Colorado Springs has a unique visitor center and chapel area that is free and open to the public. Visitors can learn a wealth of information about the academy and it's history as well as view the planes on display. The Chapel is absolutely beautiful and due to it's unique architecture it's probably the most photographed building in all of Colorado.
Olympic Training Center Tours in Colorado Springs (always free)
The United States Olympic Training Center in Colorado Springs is the home of a huge Olympic visitor's center, the U.S. Olympic Hall of Fame, and serves as the training center for many of our country's top athletes. A free tour of the Olympic Complex is one of the best things to do in Colorado Springs.
Lookout Mountain Nature Center (always free)
This free nature center on top of Lookout Mountain in Golden is one of the top things to do in Colorado with the kids.