There's a lot to do in Barcelona, from cultural activities to just relaxing in the cafes and the bars. Tha'ts if you're not spending the whole day by the beach of course.
If it is your first time in Barcelona, you may want to try the touristic bus, which is nice but won't be able to take you to the Gothic cathedral (see photo below) as it is in the middle of a pedestrian area.
You can also easily find your way around by tube but when the weather is looking good, which is most of the time in Gaudi's city, then travelling outdoors is definitely your best option.
https://www.airbnb.co.uk/experiences/160261 - Ride the city on an electric scooter for almost two hours with airbnb
How do you travel from one monument to another ?
Unfortunately you can only get a city bike Bicing if you are resident in Barcelona, so you will have to rent a bike or anything else for that matter from a tourist rentals shop or try to book a city tour through airbnb : One way ride from Park Guell to the Rambla in style !
Three days in Barcelona :
Here a couple of really good ideas if you are currently organizing a few day in Barcelona. Simply follow the links and see what the city has to offer :