Planespotting in Barcelona
One of the best airports for planespotting I have ever been to!...there are even two purpose-made viewing platforms between the runways and the beach (In this map you will find the approximate location of, what I think, is the best place for planespotting at Barcelona airport, and in this other one (good find, Pau Cuevas!), there is comprehensive guide of the best locations for spotters at Barcelona airport)
But there is a catch: the planespotting platform is not easy to get to unless you have a car (there is a local bus, but, as far as I am ware of, it does not connect with the airport terminals but with the train station at the nearby town of El Prat).
Another factor to take into account is that most of the traffic at Barcelona is to and from European destinations, which means that Airbus A320 and Boeing 737 will account for an overwhelming majority of the sights.
But you can still spot some visitors from afar, like this Pakistan International Airlines Boeing 777-200 or a Qatar Airways Airbus A330!