There are many summer activities within the resort such as hiking, mountain biking, ATVing, and golfing. Closer to the date, a link will be available with a group discount code for these activities
Here is a compiled list of easy hikes in the immediate area- not all have been hiked personally so use caution, all trails does tend to exaggerate the ease and underrepresent the distance of some hikes.
Red Rocks Park & Amphitheater, RiNo Art District, Meow Wolf, Downtown Golden, Golden Gate Canyon State Park and Panorama Point, Eldorado State Park, Downtown Boulder, the Flatirons
Nottingham lake, Explore Vail and Beaver Creek villages, Vail Gondola, Betty Ford Alpine Gardens
If you are making a trip out of it, here are a few colorado gems that are worth visiting: Rocky Mountain National Park, Brainard Lake Recreation area, Telluride/Ouray and the San Juan mountains, Buena Vista, Leadville, Garden of the Gods