Just keep in mind that Barcelona was once a medieval walled city. The walls have come down, but it remains a relatively small city where walking is the most interesting and most expedient option, especially in the narrow streets of the old city. That said, Barcelona has a number of other efficient and relatively inexpensive ways of getting around the city.
Major rental car companies are located all together in Terminal 1 and 2, giving you an opportunity to shop for best deals; but it's likely more economical to book online ahead of your arrival. They are open 7 am to midnight.
Residents get around on a bike-rental system called "bicing," but you can also find hourly rentals at cycling shops and a wide selection of bike tours, some including tapas and electric bikes.
The Metro system is clean, efficient and more or less punctual. It can get you most everywhere a tourist would want to go with the same ticket working for both Metro and buses. It is air-conditioned, a big plus in the summer even if you had planned on walking a short distance. The Metro system connects to RENFE trains that will get you to/from the airport in under 30 minutes for €4.10. If you're arriving at Terminal 1, you'll need to hop the shuttle to Terminal 2 first.
With more than 1000 buses traveling on 80 routes, tourists can get almost anywhere by bus, using the same pass as the one used for the Metro. the tram, city lines of the FGC trains and zone 1 of the Renfe trains. Most service begins at 4:25 am and ends at 11 pm. Night buses begin or stop at Plaça de Catalunya between 10:40 to 11:40 pm and 5 to 6 am.
Taxis are an affordable option. Figure €10 for a 15-minute ride based on what's on the meter. Inside the cab is a rate chart. You don't have to tip, but 5-10% would be sufficient. You know that the official yellow-and-black taxis are free if the green light on top is lit. But if headed to the center of the city with its narrow and/or pedestrian-only streets, taxis are not the best choice. Try walking from the closest main thoroughfare or metro stop. For the time being, Urber is only good for food deliveries.