Best known for its unique acoustics and unforgettable concerts, Red Rocks amphitheater is also free to the public when there aren't shows playing. It's worth a visit if you've never seen the fault line that was made into a concert hot spot. There are several hiking trails, and a small museum. And it's always fun to test your fitness by trying to run up all the steps!
If you have a lot of extra time out in Colorado, there are plenty of places to try out your ski legs. There is Vail, Winter Park, Loveland, Keystone, Copper Mountain, and so many more resorts. Getting out there can be tricky, but it's worth it if you have the time and want to make a long vacation out of your trip!