Rent your car at Girona station
If you will be in Barcelona city for a few days before your Girona trip, then rent your car at Girona. Rather than driving to Girona for a day trip, it will be more convenient to take the AVE and rent your vehicle from Girona station instead. And you will save the hassle and cost of parking while you are in Girona for the day.
Or rent at Barcelona airport
If you plan to start your Costa Brava driving holiday with no intention of staying at Barcelona, renting a car at Barcelona airport may work out cheaper. You can return it to the same location and avoid paying a charge for renting in Girona and drop off at a different location. It will also be more convenient as airport car hire counters are open 24 x 7 unlike station counters in Girona which work typical Spanish office hours. And don’t worry about driving from Barcelona airport to Girona; the traffic is well regulated and signposted.