The list of fun things to do in Denver in the summer is extensive! If you plan on extending your trip at all to go into the mountains (Breckenridge, Vail, Aspen, etc.), we highly recommend spending one day in Denver first to adjust to the altitude. Fun fact, alcohol affects you differently at altitude (1 beer at altitude = 2 beers at sea level). If you need any more suggestions, please reach out!
If you are up for an adventure, we recommend visiting Rocky Mountain National Park! There are many great hikes in the park, but please note that some hikes/activities require you to obtain a visitor's pass for entry. There is also a scenic road to drive along--Trail Ridge Road.
Explore a wide range of gardens and collections on Denver Botanic Gardens' 24 acres. The gardens reflect an ever-widening variation of plants from all corners of the world.
Garden of the Gods Park is a registered National Natural Landmark featuring dramatic views. You will admire the 300 feet sandstone rock formations against the backdrop of snow-capped Pikes Peak and brilliant blue skies.
If you're staying in Boulder, the Flatirons would be a great activity to explore right in Boulder. There are a variety of hikes from Easy to Hard for all skill levels. The link provides a list of the Chautauqua trails.