If you are extending your stay, you have a lot of options outside of Denver to consider. Colorado Man listed some good ones. My suggestion would be to check out what mountain resorts might have a nice event that weekend, Most have beer, wine, music, and/or food festivals throughout the summer. Breckenridge, Keystone, and Winter Park are about an hour from Denver. Vail and Beaver Creek are about 1.5 to 2 hours from Denver.
Aspen is about 4 hours from Denver and Steamboat Springs is about 3 hours from Denver, which might be a little further than you want to go for your first time in the area.
Overall, my recommendation would be to head up to Vail with your wife and stay somewhere in town. You will be a hero with her if you do this. Since you indicated that you don't want to climb mountains, you can take the gondola up to mid-mountain in Vail and have lunch at the chalet at the top of the gondola called The 10th. Beautiful views of the Gore mountain range from there, and then you can take the gondola back down and enjoy the town of Vail as well. Have a great trip!
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