Driving around the Costa Brava region is a good choice. With patchy transport schedules between small towns and villages, it can be difficult to get around Costa Brava using public transport. I like to say that a 4 day self-driving trip around Costa Brava is equal to 8 days without a car.
Girona has many, safe parking options
At the Station
On a day trip to Girona, you don’t need a car to drive within the city. So your best and safest parking option is at the Girona railway station which is a 15-minute walk from the heart of the old town. Try and arrive early if you are driving in from Barcelona so that you can get a parking space.
Across the river - free
If you cannot find space at the railway station, head to the Mercadona supermarket to the north of the river Ter, off Av de Franca. There are also lots of open-air car parks further up the road in this area including at Carrefour, Ibis Hotel, and Hotel Costabella Girona.
Old town - paid
And if you arrive really early you can actually get free streetside parking in the Girona old city as well, there is no harm trying this first before looking for other options. This will save you 30 minutes of walking time that you can use for visiting Girona sights during your day trip. The old city also has Hotel Palau de Bellavista (on a hill) with a paid parking. Another option is Montjuic B&B on the east of the Riu Ter; it is a 20 minutes downhill walk from this parking to the Cathedral.
And rest assured that you will not have a safety problem even if you decide to park until late in the night or even overnight. Here is a parking map for Girona which basically highlights the areas that I mentioned.