I'm finalizing plans for an early April trip to Andalusia. Flying into Madrid seems to be the most cost effective but, because it's Spring, I want to start south and work north - so first thing will be getting from Madrid to Seville (probably by Ave train). From there it seems reasonably easy to get to other Andalusian towns without a rental car but I'm wondering about Gibraltar, is there a good option from Seville to Gibraltar without a rental car?
It could also be Granada or Cordoba to Gibraltar, I can plug the leg in where it works best.
There is reasonably good bus service throughout that area. You can get schedules here:
http://www.movelia.es/venta/asp/index.asp although the ones for next year may not be available for a while. For this month there are 4 buses/day from Sevilla to La Linea de la Concepcion. The trip is about 4 hours and the fare is €21.20. The La Linea bus station is only about 200 meters from the Gibraltor crossing. Movelia can also give you connections to many other places.