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.
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.
Louisville Street Faire
The downtown Louisville Street Faire in Louisville, Colorado brings great music to the Front Range every summer Friday from mid-June to mid-August. This fair's last performance of the year is usually the biggest with internationally known headliners.
In addition to the concert, the fair features excellent food and drink vendors, and children's activities such as arts and crafts.
When:
Summer Fridays from 5 to 10pm.
Music is typically 6:30 - 9.
Where:
Downtown Main Street in Louisville.
Schedule:
The 2013 Louisville Street Faire schedule is as follows:
June 14: CJ Chenier
June 21: Young Dubliners
June 28: Richie Furay
July 12: BoDeans
July 19: The Samples
July 26: Samantha Fish
August 2: Tab Benoit
August 9: Fireball
August 16: Dickey Betts and Great Southern (CANCELLED DUE TO ILLNESS)
I was really looking forward to seeing Dickey Betts again and hope he gets well soon and can come back to Colorado.
Chris Daniels and Freddi will be stepping in at the last minute for this concert.
More Info from the downtown Louisville web site: 
Punkin Chunkin Colorado | Aurora Jack-O-Launch (annually, 2nd Saturday in October)
This October festival has become a Denver tradition.
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...
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!
Clear Creek History Park, Golden (always free)
Clear Creek History Park in downtown Golden is now free to walk through, sunrise to sunset every day.
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Denver Firefighters Museum Free Days
The Denver Firefighters Museum free days are a fun way to learn about real Colorado firefighters and the history of fire fighting in Colorado.
University of Colorado Museum of Natural History at Boulder (always free)
The University of Colorado Museum of Natural History is a free natural history museum on the CU Boulder campus. They have excellent changing exhibits, a dinosaur section, and an extensive bird display. A small donation is requested to help keep the museum open and free. This is a fun small natural history museum for adults and curious kids.
When:
Monday - Friday 9am - 5pm
Saturday 9am - 4pm
Sunday 10am - 4pm
Where:
The Henderson Building on the CU-Boulder campus, between 15th and 16th streets east of Broadway.
For details check their web site:
The Littleton History Museum (always free)
The Littleton Museum is a 14 acre indoor and outdoor history museum depicting Colorado pioneer history. Two historic farm sites featuring a schoolhouse and blacksmith shop bring history to life in a way that consistently rates this museum as one of the top living history museums in the United States.
Shell Gas Two For One Ski Ticket
For 2014, Shell gas stations are offering 2 for 1 vouchers at several ski areas with a 10 gallon gas fill up. This means that if you buy at least 10 gallons of gas, you get a coupon good for a free lift ticket if you purchase one full price adult lift ticket. The deal is good at several ski areas.
Unfortunately, full price adult lift tickets have become so expensive that this isn't much of a "deal". But if you just want to ski one day at a participating resort this year, this will get you on the slopes for less than full price.
In Colorado, 2014's participating resorts are:
Copper Mountain
Crested Butte
Loveland
Monarch
Powderhorn
The SkiFreeDeals.com web site has a full list of participating Shell stations and ski resorts.
Learn more at skifreedeals.Com:
Liftopia Ski Discounts
Colorado resorts offering substantial Liftopia ski discounts:
Arapahoe Basin.
Copper Mountain.
Durango.
Loveland.
Monarch Mountain.
Ski Cooper.
Sunlight.
Winter Park.
Some other resorts have discounts on Liftopia that really aren't discounts at all.
Check out Liftopia:
Rentals
If you need to pick up rentals on your way to the mountain, my favorite super cheap rental shop is Maison De Ski in Idaho Springs.
Colorado Railroad Museum in Golden (free days)
The top Colorado attraction for train enthusiasts also has free days! A few free days are held each year at the Colorado Railroad Museum in Golden. This museum features a diverse collection of trains, some restored and some not, located outdoors. You can climb aboard some of them. There are also indoor museum exhibits and a huge indoor miniature train layout that is operated by quarters.
Denver Museum of Nature and Science (free days)
One of the top things to do in Denver with older kids, the Denver Museum of Nature and Science is a hands-on educational museum that kids love!