Old Globeville Days and the Orthodox Food Festival (annually in July)
The Old Globeville Days Orthodox Food Festival celebrates Denver's international immigrant heritage with a diverse selection of free entertainment and international food. The music features outstanding performers of diverse folk heritage including an amazing Celtic performance. The incredible food selection is sample sized and you purchase tickets to try the samples. One of the best things to do in Denver on a warm mid-July weekend.
Movies In the Park
Looking for a popular Denver attraction for the kids? The free Denver Movies in the Park are one of the top things to do in Denver with the family, and not to be missed if you're in Denver with kids.
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... 
Golden Buffalo Bill Days (late July)
Golden hosts an annual festival celebrating the life and spirit of Buffalo Bill, who is believed to be buried at the top of Lookout Mountain. If you're in the front range during the last weekend of July, Buffalo Bill Days is one of the best things to do on a warm summer weekend.
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.
Sunnyside Music Festival (mid-September)
The free Sunnyside Music Festival is a showcase of Colorado talent that takes place each year in Denver's Sunnyside neighborhood.
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.
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Boulder History Museum (free first Sunday monthly)
The Boulder History Museum offers free admission on the first Sunday of every month. This is a great deal and a chance for the rest of us to check out the museum's permanent displays, changing exhibits, and kids activities.
Smithsonian Free Museum Day
Every year some of the best museums in the USA take part in the Smithsonian Museum Day.
Underhill Museum | Idaho Springs | Always Free
The free Underhill Museum is a small but quaint depiction of early life in downtown Idaho Springs, with several rooms set up to display a typical home setting in the early 1900's. The kitchen in particular is very elaborate, and during better weather the Victorian Garden is open to the public. In addition, there is an old vault, antique cash register, and many photos of Idaho Springs through the years.
This free museum earns revenue for the History Society of Idaho Springs with a bookstore located at the entrance. They have an excellent selection of Colorado books as well as a large used book inventory.
This is a nice free historical museum to visit while browsing the shops in downtown Idaho Springs. I'm no fan of tourist trap shops that are full of cheap imported junk, like many of the shops in more popular Colorado tourist destinations. The shops in Idaho Springs are not like that. Many feature locally made artisan works for reasonable prices.
Fall and Winter hours 2011-2012:
Saturday, Sunday, and Monday 11am to 5pm.
Location:
The Underhill Museum is at the western end of the strip in downtown Idaho Springs on Miner Street. Look for the sign that says "Books".
While you're in the area check out the Visitor Center and Heritage Museum at 2060 Miner Street.
Historic Idaho Springs 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.
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.
Half Fast Subs (Boulder)
Half Fast Subs in Boulder has one of the largest menu choices I've ever seen at a restaurant. The hardest thing to do at this place it to make a decision between the hot subs, cold subs, veggie subs and even salads. Whatever you chose, you won't be disappointed.
The Hornet Denver Happy Hour (SoBo district)
When perusing the happy hour menu at The Hornet in SoBo, just south of downtown Denver, you really only need to ask yourself one question- What will I order with my Bacon and Brie Home Fries? Yes, you read that right, you can order a plate of home fries piled with creamy brie cheese and salty bacon. What else can a person want?