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... 
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.
Wheat Ridge Greenbelt (always free)
The Wheat Ridge Green Belt, also known as the Clear Creek Trail, is a beautiful area in the Denver Colorado suburbs to hike, bike, or just play on the jungle gyms!
Free Summer Bowling For Students at Brunswick Bowling Centers
Brunswick Bowling Centers are offering free bowling for students during the summer of 2012. Kids or their parents (depending on age) sign up for Brunswick's Bonus Zone to get free bowling.
There are a few Brunswick Bowling Centers along the front range that offer the free bowling for kids promotion.
Find a bowling center:
Bonus Zone Sign up:
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!
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.
Buffalo Bill's Birthday (February)
Golden, Colorado celebrates Buffalo Bills birthday in grand style every year at Rock Rest Lodge. This free festival features re-enactments of the wild west days as well as great music, food, and western authors.
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Denver Restaurant Week 2016 (Feb 26 - March 6)
For 2016, Denver will be celebrating Restaurant Week in February and March. Denver Restaurant Week features top downtown restaurants for a reduced price.
Coors Tour: Golden, Colorado (always free)
The Coors tour is one of Colorado's most famous free tours, and one of the top Colorado attractions. If you are in the Denver area, try to make the trip west to the Coors tour in downtown Golden.
After parking, you will be picked up by a small bus and taken on a short tour of downtown Golden. The driver will provide information about Golden and Coors along the way.
After exiting the bus, you go into the Coors tour center, where you will receive an audio device that you listen to as you follow the self-guided tour.
The Coors tour is very in depth, taking you through the history of Coors and the stages of brewing and packaging. You will see the huge brewing vats and other facilities critical to the process. The packaging area is one of the most dramatic aspects when it is in full operation.
In the middle of the Coors tour you enter a room which serves a small sample of the freshest beer you will likely ever taste, recently off of the production line. At the end of the tour you enter the bar area where you receive three free drinks of your choice! And don't worry, they have root beer and other drinks for the designated driver and the children.
The Coors tour is so popular that it can be very crowded with long wait times at the bus and bar. Consider attending earlier in the day or on a weekday.
When:
Winter Hours:
Thursday 10-4
Friday 10-4
Saturday 10-4
Sunday noon-4
Monday 10-4
Closed Tuesday, Wednesday and all major holidays.
Summer Hours (May 29 through Labor Day):
Monday - Saturday 10am - 4pm
Sunday 12noon - 4pm
Closed on all major holidays.
Specific Closure Dates:
April 20th.
May 26, 27, 28.
July 4th.
September 1, 2, 3.
November 28-29.
December 24-25.
December 31-January 1.
Where:
The Coors tour is located in the middle of downtown Golden. The gigantic concrete buildings are impossible to miss.
Parking for the tours is in a large lot at the intersection of 13th street and Ford Street. The line for the Coors tour bus forms near this intersection.
Information about the Coors tour from the MillerCoors site:
Colorado Ski Museum & Snowboard Museum at Vail (always free)
The Colorado Ski Museum & Snowboard Museum in Vail Village celebrates Colorado's rich ski history with a huge collection of artifacts and exhibits covering the whole of ski and snowboard history in Colorado.
Children's Museum of Denver Free Days (first Tuesday monthly, 4-8pm)
One of the best free Denver attractions for kids, the Children's Museum of Denver's free day takes place from 4pm to 8pm on the first Tuesday of every month, known as Target Tuesdays.
Longmont Studio Tour (September 29-30)
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:
Denver Zoo Free Days
The Denver Zoo free days are by far the most popular Denver free days for families with kids, and are typically held during the fall and winter.