barcelona

It's not outside the city, but the Sant Pau modernista site is fairly new and not on a lot of people's radar yet. I'm guessing you haven't seen it.

Girona is lovely and could about fill a day even for active travelers; I thought it was worth an overnight and a bit more than a full day. Or there's the Dali Theatre and Museum in Figueres.

But since you're traveling in the summer, Montserrat might be the best choice since it should be somewhat cooler than the city.

... I would like to clarify that Ann's comment as being "fairly new", she means new as "open to the public for visits" as the site in itself was built in the early 1900s and functioned as a hospital until the early 2000s :)))

Yep. What I posted was misleading. Thanks for clarifying, Enric.

Gosh, I am annoyingly punctilious, am I not? LOL!

No, unclear posts can lead people seriously astray.

I'm taking note of these tips. :-)
Heading to Barcelona in October.

Right, now that I have some spare time I thought it can be useful to mention a few "other" sites for those that have already visited Barcelona before and hence have visited "the most visited" sites already. While it'd be very tedious gathering such a list without knowing the OP's likes and preferences, I've limited myself to name just a few:

In Barcelona...

• Torre Bellesguard - another work from Gaudí over the medieval palace of King Martí l'Humà built in 1421 located in a very posh area of the city
• Monestir de Pedralbes - a Clarisse's monastery, north of the city, dating from 1326 with a large and very beautiful cloister
• Parc del Laberint d'Horta - one of the first parks of Barcelona (dating 1792) with a nice maze
• Colònia Güell -actually at just 20' by metro from Barcelona, in Santa Maria de Cervelló - a breathtaking crypt made by Gaudí in a 1900s industrial textile colony
• Museu d'Història de Barcelona (MUHBA) - underground Roman remains of Barcelona
• Museu d'Història de Catalunya (MCAT) - superb museum of the history of Catalonia, the region where Barcelona is the capital of. Excellent for visiting with children.
• Parc de Cervantes - if visiting from May to July, this is also known as the rose garden as it hosts over 2000 different types of roses as well as the International Rose Contest every year
• Jardins del Palau de Pedralbes - located near Cervantes, these gardens belong to the royal residence of the Spanish king when visiting Catalonia
• Museu del Modernisme Català - a museum that is dedicated exclusively to Catalan Art Nouveau
• Cosmocaixa - Voted one of the best science museums in Europe, ideally for visiting with 6 - 14yo as it's a "touch everything" museum.

... and then of course, for those interested in art, there are a number of museums, galleries and foundations spread across the city.

Outside Barcelona... on an easy day escapade by train

• Sitges -the famous quirky seaside town near Barcelona
• Tarragona -former capital of the most Western province of the Roman Empire
• Tossa de Mar -another picturesque seaside town with a castle right by the beach
• Vic -worth visiting on a Saturday morning, during the open air market in the streets of the centre of the town. Aside from an important medieval city in Central Catalonia, Vic is also very famous for its sausages and cured meat ("embotits" in Catalan, the local language). Check out for brands like Duran, Riera, Bofill... the best places are the small charcuteries located on the north side of this main square, such as Xarcuteria Solà, Can Vilada, Ca la Teresona 1837 or Can Mariano, all a stone throw from each other.
• Montblanc -another very well preserved fortified medieval town

It's also imperative to check out the different festivals in cities and towns celebrated across the year: heritage and folk, food, craftsmanship, etc... as these are often the highlights for many visitors. For the city of Barcelona, this is a good resource to keep abreast: http://meet.barcelona.cat/en/ Elsewhere in Catalonia you'll have to check the city or town's website, just google: name of the city/town plus the word "turisme" and you're likely to find info on events. Most cities and towns in Catalonia have a section aimed at visitors (local and foreign alike) with info on what to see/do, etc.

Someone pointed to me on a PM that I haven't mentioned Girona, Montserrat, Cadaqués or Besalú for "sights outside the city to see"... well, those are within the list of "most visited" places already and I was trying to veer towards a list with places that don't normally appear in all guides -and if they do, they're not necessarily marked as must-see... which is totally absurd because they're superb too.

I can vouch for Torre Bellesguard and Colonia Guell, but as of last October there was quite a walk from the train station to the Colonia ticket office, so the travel time was far longer than 20 minutes. I thought it was very much worth the trip. In addition to the chapel there are other interesting buildings to see, and there's an audio guide. It wouldn't be great on a rainy day, though, because there's a good bit of walking involved.

There were occasional English-language tours at Torre Bellesguard last year. My tour turned out to be a private one, which was cool.

The Museu de Modernisme is worth it for serious modernisme junkies, but a brief visit was included in the cost of the tourist office's modernisme tour last year, and I decided I didn't need to pay the entry fee for a return visit. The collection is good, but it is not huge. Of course, modernisme junkies will also want to see the modernisme collection at the MNAC. And they should Google Ruta del Modernisme and consider buying the book/map package. Discounts on entry fees may cover the price, and the book makes a great if rather heavy souvenir.