While you have hundreds of acres of untouched countryside on your doorstep, you can also take a trip to Arapaho Nature Reserve in Walden. If you enjoy birdwatching, this is the best place in the state, inhabited by unusual species like the sage grouse and the lek, which puffs out its chest during the spring mating ritual.
Another hit with little ones is grand in scale: the giant dinosaur skeletons at Dinosaur National Monument never fail to wow. The Quarry Exhibit is especially impressive, with over 1600 bones sticking out of the rock.