The ruins at Yagul aren't as elaborate as those at Monte Albán or Mitla, but their position atop a hill makes them worth a visit. This city was predominantly a fortress protecting a group of temples. The Palacio de los Seis Patios (Palace of the Six Patios), a maze of hallways leading to hidden courtyards, is fun to explore.
If you find the eerie Tumba Triple (Triple Tomb) locked, give the guard $1 or so to open it for you. He may even let you borrow a flashlight to get a good look at the spooky carved skulls. Follow the steep trail that starts near the parking lot for a good hike and great views over the valley and ruins. The site has restrooms, but no other facilities.
Tlacolula, Oaxaca, Mexico