Where are you driving in from? When you say you are going to "spend your day at the park," be aware that the park is a long way from anywhere (1 1/2 hours from Durango, for example), so a big part of your day is going to be spent getting there and back unless you're staying on-site or nearby.

Obviously the cliff dwellings are the attraction. You can walk to Spruce Tree House from the visitor center, and that's a good place to get an overview of the park and it's history. You can go down into one of the kivas at Spruce Tree House and see what that's like. Since it's walking distance from the visitor center, Spruce Tree House gets pretty crowded.

For more in-depth experience, visit Cliff Palace, Balcony House, or Long House. You do need to purchase a tour ticket for these. Ranger-guided tours are offered seasonally. Tour tickets can ONLY be purchased in person at the Far View Visitor Center. (From Oct. 17 to Nov. 6, tickets are available at the Chapin Museum.)

http://www.nps.gov/meve/index.htm

One other tip, your cell phone won't work in the park or anywhere near the park. Turn it off for the day or your phone will wear the battery down looking for a tower. (I speak from experience.)