Free to visit, but paid parking.
Rocky Mountain National Park is a beautiful natural space. At the higher elevations, The aspens trees should be beginning to turn a beautiful golden yellow. Note that many of the hikes are vigorous, especially if you are not used to the higher elevation. Pace yourself and enjoy!
Garden of the Gods is a must-see if you make the trip down to Colorado Springs. There are lots of easy walking trails and beautiful red rock formations.
Red Rock Amphitheatre is a legendary venue - but it also has hiking trails and plenty of natural beauty. Just check and make sure there isn't a concert when you are trying to visit!