tarjeta dorada

I flew out of Barcelona but not into that city, so I can't say for sure what services are available at the airport train station. I'm sure you can get the Tarjeta Dorada at Sants, but I think your odds are good at the airport itself. If you don't encounter a tourist-info booth at the airport itself, you might look for the one at Sants. It's on one of the interior concourses, not against the wall. You can get maps there, though for a comprehensive collection of tourist brochures the offices beneath Pl. Catalunya and in Pl. de Sant Jaume are best. Across the concourse from the Sants T.O. and slightly to the left is a tobacco shop that sells T-10 tickets for the city buses and subway. One such ticket can be shared by multiple people (validate it once per person, each time you use it), and it's a very good deal.

Unless there's something really incredible going on in the city next week, I think you'll be fine to buy your La Sagrada Familia tickets after arrival. In early August last year I was told to buy mine 48 hours in advance if I wanted the earliest time-slot (which I did, since it's less crowded then), and it worked for me. In case you are not aware, you have a choice of ascending one of the towers (extra cost) or not. If you want to see one of the towers up-close, you'll want to coordinate those ticket times. I'd suggest the church first, then the tower about an hour later. If you need more time in the church itself, you can finish viewing it after the tower ascent.

Other Barcelona sights that are subject to long lines and would benefit from advance booking (after you arrive should be fine): Parc Guell, Picasso Museum (an utter zoo), Casa Batllo (also extremely crowded the day I was there, but at least I didn't have to wait in a long line to get in), Casa Mila/La Pedrera (marginally less crowded), and Palau de la Musica Catalana (tour required, available in English). Except for the supplemental tower ticket for LSF, the Pl. Catalunya T.O. was selling all those tickets last summer for a service charge of 1 or 2 euros per ticket. You can probably do everything on your smartphone, but I was more comfortable having an old-fashioned piece of paper for each site.

Barbara,

Yes, I believe you can buy the Tarjeta Dorada at the Renfe office at the Barcelona Airport, or at the Sants station for sure.

Regarding the tickets to the Sagrada Familia...I think it's best to buy in advance, but I've not been to Barcelona recently, so I'm sure others will chime in and answer your question.

Have a wonderful trip!

Edit to add:
You may find this website helpful as it explains exactly how to get to the Renfe train station at the airport.

Edit to add once more!

I just remembered this post from a few months ago which gives detailed information as to how to arrive to the train station at BCN airport.

THANK YOU all. We arrived Barcelona today and found everything. We are here a week and already wish it could be more time. Love Spain!!

Thanks for the update, enjoy the rest of your trip!