A day trip by car is fine, ELB. I suggest you start early (7.30-ish) and go via Ronda. The drive is beautiful at this time of the year, with wild flowers in abundance at the side of the road between Ronda and Sevilla and the journey should take you no more than two and a half hours, probably less.
As you come into Sevilla from this road, you will come to a football stadium on the left hand side, called the Estadio Manuel Ruiz de Lopera, which is the home of Real Betis. (Boo! :p ) I would advise you to start looking for parking on the main road anywhere around here, otherwise if you don't know the city too well it can be a bit difficult to find your way around and parking is difficult at "normal" times of the year.
There are usually men who will wave you into a free space. Although it is free parking, give them a Euro and make sure there is nothing in view in the car at all. From here you can either walk (you are near the Maria Luisa Park) or catch a passing taxi to go the remaining couple of kilometres into town for five Euros or so.
Enjoy your day - you'll have plenty of time to look around. When you walk round Barrio Santa Cruz you must call in for tapas at Cafeteria Santa Teresa and have the pork wrapped in bacon
Don't forget the Saturday morning market at Puerto Banus when you're in Marbella and Mijas is well worth a visit too. Have a nice trip!