Provided that the weather cooperates, hiking should be great.
There are interesting trails, all over Summit County.
One fun walk is Boreas Pass, and especially if the aspens have begun turning.
We just spent 6 days in Breckenridge, also visited Dillon and Frisco. Lots of hiking, biking and water activities in the area if you're into those things. You can see my complete trip report in the Colorado forum: https://www.tripadvisor.com/ShowTopic-g28927-i252-k13642545-Colorado_mostly_Breckenridge_area_Trip_Report_8_2_8_10-Colorado.html
(We're 65 and 71, though I'm not sure it makes a whole lot of difference.)
Thank you so much!!!