To maximize your time, and to give yourself to get "off the beaten track" as Adam says, stick with the idea of renting a car for Sevilla-Granada. There are so many interesting towns down through that region that you will miss if you stick with the bus and/or train. With your schedule, you've got the time to do that. Rent your car from the Santa Justa train station in Sevilla (or anywhere on the west or south side of the city, outside of the downtown maze) and you won't have any problems driving out of Sevilla. Sevilla-Ronda is only about 2 hours (stop in Zahara de la Sierra on the way), while Ronda-Nerja is about 2.5 hours (take the A-366, A-354, and A-357 through the mountains...very twisty, but a magnificent drive.) Just make sure you avoid travelling across the north of Malaga during rush hour on your way to Nerja....it can get pretty packed. Their rush hours start and end a little later than ours. Then from Nerja to Granada, it's a little over an hour up toe A-44, with the Sierra Nevada looming on your right. You may want to avoid driving IN Granada, so you can proceed west along the A-92 to the airport, drop your car there, and take the airport bus back into town.