I just did a quick radius search of what is reachable within a four-day round trip from Denver.

Within Colorado itself, you have
Rocky Mountain National Park
Dinosaur National Monument
Great Sand Dunes National Park
Canyons of the Ancients
Mesa Verde National Park
Black Canyon of the Gunnison

And you also have available some great cities and towns, such as Colorado Springs. I guess it really boils down to what your interests are.