I just stayed at the Renaissance in Barcelona by the airport. They do provide complimentary transportation to the port near the bottom of Las Ramblas during select times of day. The free transportation times are at 9:15AM & 10:15AM and return times are 4:45PM & 5:45PM. For only 5 euro/person each way you can be dropped off at noon or picked up at 8:00PM. We found that being dropped off at 10:15 and picked up at 5:45 gave us plenty of time to explore the city, exhaust ourselves from the walking and heat, and get us back to the hotel just in time for complimentary tapas and wine.