Hi Joe!
This summer I flew into Barcelona very early in the morning and had to wait till evening to pick up relatives at the train station before hopping on a long distance bus even later. I ended up having a wonderful day. There is an excellent train connection between the airport and the Sants train station. I left all my luggage in a large locker at the train station and then roamed the city, using the tourist bus as my main mode of transportation. (It stops at the main door of the station - couldn't be more convenient.) I got off at spots I hadn't visited yet, and had lunch along the way. Then I returned to the train station, picked up my luggage, and was off again - this time by taxi to the bus terminal (which, if I remember correctly, also has lockers). It was all very easy, but I did have to ask a few times where the train connections were - the signs are not very helpful. Be sure to give yourself enough travel time! There's a bit of a walk between the airport's train station and the terminal. Be sure to have luggage with wheels.