Probably the most interesting place we visited during our summer road trip was Mesa Verde National Park in Colorado, home to a large number of ancient puebloan cliff dwellings.
While several of the villages, built into alcoves eroded under the cliffs forming the edges of two mesas, are visible from the tops of the mesas, to get the best views, you need to buy tickets for the ranger-guided tours. We visited Long House, Balcony House and Cliff Palace and all were spectacular. Beware the Balcony House tour if you are scared of heights or have trouble with ladders!
Regardless of whether or not you can take a guided tour, you can access the Spruce Tree House village by foot and I would definitely recommend this too.