Lots of things to do in COS and between COS and Breckenridge. In COS, it's been mentioned: Garden of the Gods ( a Must ), Drive the Pikes Peak Hwy, Air Force Academy, Cheyenne Mtn, Boadmoor, Pro Rodeo HoF, Olympic Training Center, 7 Falls, North Cheyenne Canyon ( Helen Hunt Falls, Bear Creek Rd, Gold Camp Rd ), Manitou Springs, Old Colorado City, etc.
Someone mentioned staying in Buena Vista, I prefer Salida, better Hotel choices cool old downtown. Driving over Independence Pass is a must. stop and let the kids run explore the small Ghost Town there. We almost always see Big Horn Sheep around the pass. This time of year the snow will still be on top too. Get a snow ball fight

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I have never been to Grand Junction, but if it is not a 'have to' for you, i would recommend detouring to Aspen. You can stay on the cheap at Carbondale and drive into Aspen in 15 minutes. Plenty of parking this time year. The Maroon Bells are phenomenal to hike around. Also you can do Horse back riding at the T Lazy 7 ranch. 2 hour, half day, over night rides.
http://www.tlazy7.com/summer-activities/#horseback. Go early if you want to drive. cars are banned at Maroon Bells between 8-5, have to ride the bus. Worth it to get there early and see the sunrise hit the peaks ( sunsets too ).
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If you have time, from carbondale, head over to marble, co and take the Crystal Mill Jeep Tours. Crystal Mill is one of the most photographed spots in the Rockies.
http://smithfamilycolorado.com/CRJT/ Fun outing for the day.
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