The summer brings a lot of excellent concerts as well as a nice place to hike around.
The art museum in Denver is world class. If you go, make sure to stop by Leven Deli after for a sandwich.
Boulder is an awesome city 30 minutes from Denver. You can walk along Pearl Street, along the river, or check out their Saturday morning farmer's market. If you want to hike, there are plenty of short hikes around there including the flatirons which are stunning.
Check out the free Celestial Seasoning's tour in Boulder.
Check out Denver's first brewery, with free samples along the way.
Want to see what it's like to be at 14,000 ft? You don't even have to hike this one. Make sure to get your permit ahead of time.
Catch a baseball game at Coor's Field.