All key attractions are in Zone 1
Most tourists to Barcelona get confused with the zone system. As far as metro goes, there is only one zone system for all of Barcelona.Renfe national railways and FGC (Catalan railways) have zones. You can check the Barcelona zone map for more details.
Most of the attractions are covered in Zone 1 of Barcelona. Thus popular attractions like Sagrada Familia, Park Guell are all in Zone 1 of Barcelona. Barcelona airport is also in Zone 1. Using the T10 (Zone1) ticket, you can use RENFE train or TMB No. 46 Airport bus and the Airport NitBus N17 to travel from airport to city centre. T10 ticket cannot be used for the airport metro Aeroport T1 and Aeroport T2.
Attractions not covered in Zone 1
La Colonia Guell
Colonia Guell is a small working community made by Gaudí. There you can enjoy the incredible architecture of Gaudí. It is one of the most popular modernist sites of Catalonia. This is in the community of Santa Coloma de Cervelló. You can get there taking Ferrocarrils de la Generalitat de Catalunya from Plaça Espanya, lines S33, S8 and S4.
Sitges, the beach town and the gay capital of Catalonia. From Barcelona Sants station it takes 35 minutes to Sitges on Renfe Cercanias R2S towards Sant VicenÇ De Calders.
Montserrat is the holy mountain of Catalonia and there is a beautiful monastery on the top. It is also a great place for trekking. You can get here from Barcelona using the train and cable car or rack railway. There is a Tot Montserrat or Trans Montserrat tickets which includes everything for your trip.
Girona is a wonderful city in the north. The old town is amazing and also has one of the most preserved jewish neighborhood in Spain, called El Call de Girona. IT takes 40 minutes on Renfe from Sants to Girona.
Figueres is the place where famous painter Dalí was born. It has a museum with his name, Dalí museum. There is also a castle worth visiting. From Barcelona to Figueres is by train to Girona and then a 45 minute bus ride.
Collserola mountains behind Barcelona are nice for treks. You can take a 20 minute train from Pl. Catalunya to Baixador de Vallvidrera station and that is the starting point for hikes.
Tarragona was one of the main Roman cities in Spain. You still can visit a small coliseum and some other sites from this time. It is 1 hour away from Barcelona Sants by Renfe.