if you are coming to barcelona airport and have some times for your connection flight , maybe the best idea would be to have Montserrat Tour From Airport
which could be done between 4 to 5 hours and you would visit one of the main attractions of Barcelona.
Or if you are interested to visit the city of Barcelona from the airport you can book Barcelona City Tour From Air port
to explore the top 10 attractions of Barcelona in 4 to 5 hours. ( Sagrada Familia , Casa Mila , Casa Batllo , .. )
For both of these tours, your pick-up and drop-off could be Airport or any other location in Barcelona