The Blue Star Museums program offers active-duty military personnel and their families (up to five family members) free admission to select museums every summer, from Memorial Day weekend to Labor Day! There are participating museums around the country, including well over thirty right here in Colorado. No registration is required, but be sure to read their FAQ on how to redeem this offer, and note that it is for active-duty only. See complete details and the full list of participating museums here.
You may also be interested in the Colorado Springs-area list of military discounts!