If you’ve come to visit Barcelona and want to go a bit at your own pace without becoming dependent or transportation schedules, you can rent a car at the airport or in the city to go to interesting places outside the city like other beaches out of Barcelona.
If you are in one of the hotels in the city you can find information in their own reception, probably have brochures. If instead you stay in one of the apartments Barcelona, it might be interesting telling you at the airport and even rent it.
Furthermore, in Barcelona in August allowed to park free in designated areas, they are what they call green areas for residents and parking area usually blue. However, in August both are free and you just have to make sure the meter to enter the free parking area.
A taxi from the airport to downtown Barcelona, is about 25 and 30 euros you can get away with much more profitable then rent it and drive it to the apartment in Barcelona and then you can go to other places of the city.