This site is a little different from the others we visited because it's not run by INAH, but by the Mexican Institute of Culture. It's not quite as protected and we were allowed to explore whatever we liked at our own risk. It's a large but simple site and we loved it.
It closes at 5 and we got there at 4:10. The ticket seller told us that we wouldn't have time to go up since they make everyone leave at 4:30 (he really didn't want us to go up), but we went anyway. It was a quick climb up the hill and no one told us to go down and we left a little after 5 ahead of another group. The ticket guy was gone so I'm not quite sure what the problem was.
This is only about an hour from DF and I think it would make a great, low stress day trip.