If travelling by car, it really depends on how long you want to take. Give yourself six hours for the trip. That means there's not too much time left in the day for distractions, so as the previous poster said, it has to be Cordoba. See the Mosque, wander around, and if you still have time a tiny detour to the ruined Moorish city and palace of Medina Azahara a few kilometres out of town would make for a rewarding visit. If you really set off at the crack of dawn and there's still light and time you could do worse than popping into Carmona. It's a picturesque town with a stunning Roman necropolis. Have fun and enjoy the view as you pass through the Despeñaperros (the mountain pass between Castilla and Andalucia) There's a little bar perched on the abyss which is a great stop for a coffee. Have a safe and fascinating trip.