If you're traveling on a budget, you'll be pleased to know there's plenty of free things to do in Barcelona. Some museums have free entry every day of the week, while others are free on specific days (usually Wednesdays and Sundays). These are the best free things to do in Barcelona.
Of course, paying the entrance fee does at least help guarantee that they finish the building (over 120 years and counting, so far), but if you're on a very tight budget, you can still appreciate 90% of the building from across the road.
Also on Las Ramblas is the Boqueria market (though the fruit and vegetables come at a price, wandering around the stalls costs nothing) and the Plaça Reial can be found just off the main street (complete with some Gaudi designed lampposts).
Read more about Las Ramblas.
3. Parc Guell
See pictures of Parc Guell
The museum is also free to under-16s and to study groups (only on Wednesday afternoons). Inquire at the museum for more information.
On certain days there are free guided tours of the collection, but you have to pay the normal entrance fee to see it.
Read more about the Picasso Museum in Barcelona.
One of two moutains in Barcelona (the other being Tibidabo), Montjuic has a wealth of sights for those who don't mind a bit of a climb. Take a walk with a nice view of the sea, wander round the old watchtower and marvel at the Mayor's Belvedere, a collage of broken bottles and pottery by Carles Buïgas. The Museu Nacional D'Art de Catalunya is free to under-15s, over-65s and to everyone on the first Sunday of the month.
Read more about Montjuic
Though you might have to side step the elderly and infirm to get your photos, the vast majority of the hospital is open for you to wander around.
If I ever get sick in Barcelona, I'm coming here!
See pictures from Hospital de Sant Pau
Take the kids for a run in a very pleasant park in the center of Barcelona. It features Barcelona's Arc de Triomf (much nicer than the one in Paris) fountains, museums (not free) and a zoo (also not free).
Read more about the Parc de la Ciutadella in Barcelona
There is also a nudist beach here (a bit of a walk from the main touristy area).
Entrance to the Barcelona Cathedral is free, so you can marvel at the Romanesque cathedral from both outside and within. Read more about Barcelona Cathedral.