An alternative option to the bus or train could be for you to arrange car hire. You can then drive along the coast and stop off at some of the beaches en route. Find a car hire company that will allow you to have a one-way trip. saving you from having to return the car to your pick up point.
My recommendation would be to ring Girona airport information desk and ask about the car hire services or speak directly to the car rental companies - details below.
If you use an online reservation service ensure that you can book the car at one destination and drop it off at another - unless you are aiming at keeping the car for a round trip.
Hertz Car Rental
Tel: +34 97 296 6096
Avis Car Rental - Gerona Airport
Tel: +34 90 209 0249
Europcar Car hire
Girona Airport Tel: +34 91 150 5000
Girona Railway Station Tel: +34 61 142 3744