Punkin Chunkin Colorado | Aurora Jack-O-Launch (annually, 2nd Saturday in October)
This October festival has become a Denver tradition.
Frozen Dead Guy Days in Nederland (Annually in March)
2014's Frozen Dead Guy Days will be celebrated March 7 through March 9. The highlight of the event is really the Polar Plunge, where anyone can sign up to take a dive in a freezing pond. Some people do it for the thrill, some do it to raise money for charities, and some are just plain nuts. There is also a coffin race, hearse parade, music, and many other bizarre activities (frozen turkey bowling???). This is one of the most popular things to do in March for front range residents with cabin fever.
This is probably the wierdest festival I've ever heard of, even wierder than Manitou's fruitcake toss or Leadville's ski Joring. Come and learn about the history of the frozen dead guy and observe some of Nederland's finest citizens enjoying a day in the mountains.
Traffic can become a real problem for the small town so plan accordingly. Parking is available on the side streets but getting to them can take a while.
When:
March 7 - 9, 2014
Where:
The event is held in Nederland, Colorado (at the top of Boulder Canyon west of Boulder).
As more people down in town learn about this festival every year, it has grown from a small event for members of the mountain community to a really crowded winter festival for everyone in the front range. If the crowds don't bother you, come check it out:
Frozen Dead Guy Days Nederland Chamber web site: 
If you want to learn more before you commit to a trip to the mountains, read all about the frozen dead guy here:
Frozen Dead Guy Days Wiki:
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.
The Boulder Creek Festival on Memorial Day Weekend
The Boulder Creek Festival takes place every Memorial Day weekend in Boulder along Boulder Creek, and features great free music, dancing, rides for the kids, and other entertainment. This is one of the best festivals of the year, and a great thing to do on Memorial Day weekend.
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
Colorado Dragon Boat Festival (end of July)
Held annually at the end of July, the Colorado Dragon Boat Festival is a colorful and tradition-filled celebration of Asian culture. The adult and youth boat racing competitions have grown every year and are an amazing event to watch. The performing arts stages feature the talent of Asian dance and music performances as well as martial arts exhibitions. A colorful and amazing festival and a great thing to do on a summer Denver weekend.
Don't miss the 2016 Colorado Dragon Boat Festival!
Bluff Lake Nature Center, Aurora (always free)
Bluff Lake Nature Center is a free wildlife and nature center park in Aurora in the Stapleton area. Hiking trails wind their way through the park, passing a seasonal lake and overlooks. This is an excellent Denver area attraction for nature watchers.
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Colorado Sports Hall of Fame (always free)
The free Museum at the Colorado Sports Hall of Fame at Invesco Field showcases the accomplishments of Colorado's finest athletes across many sports disciplines from the most popular spectator sports to little-known gems.
There are exhibits featuring many of the greatest triumphs in Colorado sports history, from the Broncos to Golf to women in sports, as well as an interactive Kid's Zone. There is also an exhibit honoring the National Sports Center for the Disabled and the Special Olympics.
Hours
September thru May: Thursday -- Saturday 10am to 3pm
June, July, August: Tuesday -- Saturday 10am to 3pm
Beware of special hours and closures before or during Broncos home games, contact the Museum to confirm hours.
Location
Invesco Field Gate #1 (western side)
For the latest info, check the Colorado Sports Hall of Fame web site:
Colorado School of Mines Geology Museum in Golden (always free)
If you are spending a day in Golden and looking for free attractions, The Colorado School of Mines Geology Museum has a large display of rare gemstones, and their mining display downstairs should not be missed! One of the main attractions at the School of Mines Geology Museum is a moon rock collected during the Apollo 17 lunar mission!
When:
Monday - Saturday 9am - 4pm
Sunday 1pm - 4pm
Where:
1310 Maple St., Golden, CO 80401
The Museum is located in downtown Golden on the Colorado School of Mines campus, in the General Research Library building.
Check their web site for details:
Leanin' Tree Museum, Boulder (always free)
Over the years Ed Trumble has assembled what is likely one of the finest collections of western art in America. His collection is housed in the Leanin' Tree Museum and Sculpture Garden of Western Art in Boulder, Colorado.
The free Leanin' Tree Art Museum is one of the best museum values in Colorado. The exhibits are truly amazing and represent many decades and styles of western American art. The museum contains over 250 paintings and 150 sculptures.
Hours:
Monday - Friday 8am to 5 pm.
Saturday, Sunday, Holidays 10am to 5pm.
Closed Easter, July 4, Christmas Day, New Years Day.
Location:
6055 Longbow Drive, Boulder Colorado, 80301.
Located near the free Celestial Seasonings Tour.
Check the Leanin' Tree Museum site for the latest info:
Santa Fe Art Walk (first Friday monthly)
Denver's Art District on Santa Fe Drive features one of the more diverse collections of galleries in the United States, all situated within a short walking distance.
Patsy's Candies Tour in Colorado Springs (always free)
Patsy's Candies in Colorado Springs takes the making of chocolate candy to a whole new level. Free tours of their factory demonstrate the handcrafted nature of Patsy's Candies and highlight the artisanship present in their high quality candies.
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.
Aurora History Museum (always free)
The Aurora History Museum is free and open every day except Monday and certain holidays. There is a permanent display of Aurora history as well as changing displays. This is a great Denver area free attraction, where you can gain a perspective on the real Colorado and the early days of settling this landscape.