My TOP 3 list of excursions as a day trip from Barcelona
Go to Montserrat if there is only one day trip you can make as an excursion from Barcelona. I have seen that two thirds of my guests end up picking Montserrat as their top day trip excursion choice.
Highlights of Montserrat - Montserrat has a beautiful monastery and basilica that is located in the stunning jagged hills. There are many hiking trails too, and the boys choir is fantastic. If you are looking for natural beauty then this is the place for your excursion. But apart from this there is not much else for Montserrat to offer on an excursion from Barcelona.
Getting to Montserrat - Choose between the Trans Montserrat pass and TOT Montserrat pass as the best way to visit Montserrat as an excursion from Barcelona.
Choose Girona for your excursion from Barcelona If you want a mix of things to see with the feel of a Catalan small town. Barcelona is certainly not what the real Catalonia is like; so an excursion from Barcelona to Girona stands out even more!
Highlights of Girona - Personally I prefer going on a 1 day trip excursion to Girona over Montserrat because it is a university town with a history, world class food, a young vibe, and sophistication. Also the Game of Thrones was extensively set in the old town and Cathedral of Girona.
An excursion to Girona from Barcelona can easily be done in a day, with the option of adding another day excursion to the best beaches of Costa Brava. If you have more time, the Dali Triangle is another major attraction.
Getting to Girona - The AVE train from Barcelona Sants gets you in Girona, in just 40 minutes.
My guests that like glamour, style, and beaches always pick Sitges as their excursion choice from Barcelona. Sitges has a lot more than beaches!
Highlights of Sitges - Many people associate Sitges with being a LGBT travel destination and yes you will certainly find that the buzz of the place is greatly increased because of the diverse crowds. This is what makes the beaches of Sitges a welcoming excursion from Barcelona, for everyone including families.
Getting to Sitges - From Barcelona Sants, trains take only 30 minutes to Sitges. If you are going directly from the airport as many visitors do, a MonBus takes under 30 minutes from Barcelona airport to Sitges.
Less known must-consider excursions from Barcelona
If you like the thought of a beach town excursion from Barcelona, I suggest that you read this list of the most delightful seaside towns in the area.
Also consider excursions from Barcelona to the towns listed in the response by catalan_girl. Of those I particularly recommend Baga and Cardona as my top pick for excursions from Barcelona.
Popular day trips from Barcelona
Amazing and charming monastery located very close to Manresa. Undoubtedly, this is a historical enclave that brings together beautiful features of traditional Catalan architecture.
Reaching San Benet
You can go by Renfe Cercanías, line R4 (Barcelona Sants-Manresa).
If you like the artist Gaudí, you must visit this imposing and beautiful crypt. It is located in Santa Coloma, very close to Barcelona.
Reaching Colonia Guell
You can go by train (FGC), lines S33, S4 and S8.
Your chance to discover an authentic medieval village.
By bus - The nearest station is Pobla Lillet.
By train - The nearest station is Puigcerdà (Station information: 902 432 343)
From Barcelona it is very easy to visit the spectacular mountains of the Pyrenees. There are a few worthwhile stops, such as the beautiful medieval town of Vic (the traditional market is incredible) and the small village of Queralbs, with only 200 inhabitants.
It is one of the most emblematic wine regions of Catalonia. You can make different visits to wineries, along with private tastings.
Reaching El Priorat
To go to El Priorat, at the Sants station there is a train that takes approximately two hours (it arrives at the Marçà-Falset station). The line is Barcelona-Caspe-Zaragoza.
In this beautiful medieval town is one of the largest salt deposits in the world, and is called "la Montaña de Sal” Salt Mountain.
To go to Cardona there is an ALSA bus that takes about two hours, and costs about 13 euros.
Go to Figueres or Montserrat if you have a day to spare in Barcelona.
If you like art - FigueresSalvador Dalí in Figueras, is worth 14 euros entry. By train it takes only about 50 minutes from Sants station to Figueres. Don’t miss the Dali jewels display, in a separate building on 2 floors, that can visited independently. It is truly unique and precious.
In Figueras I also recommend to visit the toy museum El Museo del Juguete, with a multitude of pieces, some very old. If you like watches and clocks, I recommend visiting the Martí Amiel Collection, a very unique private collection of over 3000 pieces.
If you like nature - Montserrat
There is a train that leaves from the station of Plaza Espana in Barcelona and then you have to take a cable car that takes you to the mountain. It is an unforgettable trip. Also highly recommended the funicular that goes up and down the mountain.
For sophisticated seaside - SitgesSitges is a beautiful place if you like the sea, as it is surrounded by it. It hosts plenty of festivals such as the renowned Cinema Festival (for fantasy and horror) in October, or the Gay Pride parade. The gastronomy is also very good, the rice is amazing, as well as their wines. I do recommend it if you just want to relax and laze around without any active plan.
Go to Girona (Costa Brava)
You must go to Girona and Costa Brava if you have a day to spare. I agree Montserrat is a top choice but if you have another day, go to Girona.
Girona is a lovely university town, and many Game of Thrones episodes were shot here. And it has history, architecture, and great food.
If you are a cycling person, then Costa Brava has the best routes in Spain.