The Centro Historico area is easily visited in that time frame - take the subway to the Zocalo, wander along Calle Tacuba to the west end of Alameda Park and pick up the subway again at Hildalgo.

Visit the Palacio Belle Artes and several other small museums including the Museum of Torture, a fond trip down memory lane for Catholic school students.

Lunch (or breakfast) on the roof of the Best Western Majestic hotel overlooking the Zocalo is great. Other places of interest nearby are the Templo Mayor and the Cathedral; both at the Zocalo.

Leave time to return to the airport as traffic picks up significantly in the afternoon.