We are staying at the Almanac hotel in late September, and renting a car immediately following to drive to Costa Blava. What is the closest car rental agency to the Almanac? Will any of the agencies deliver the car to our hotel? And, are we able to drop off the car at the airport in Barcelona?
I suggest starting with Google Maps and using the search term "car rental near Barcelona" or even more specifically near your hotel. Major agencies like Europcar, Hertz, Avis, Budget, Sixt, and Enterprise each have several locations in the city and no problem to drop the car at the airport later. Enterprise might take reservations to pick you up, but most others will require you to make your own way to their office.