Would also recommend the aquarium - down on the water front. At the top of Passeig de Gracia, there used to be a small cinema - three screens - that specialised in original versions. They used to do late showings,starting after 11 pm which were great fun.
The Miro Foundation is pretty spectacular, too - good cafe. Never used the funicular but think that might be a fun way up and down Montjuic and if you have a head for heights, there is the cable car.
The Sagrada Familia is really out of this world, too and I liked Parc Guell - lots of space for running round, nice little cafe, and loads of Gaudi.
Yes, have been in Barcelona in August and it is very hot but like all cities, it slows down in August - lots of people away on holiday - and is very enjoyable.
Places to eat - el quatre gats; el julivert meu; loads of great cafes; watch out that any fish is fresh!
You will have an a ball - most . Oh, if you can be bothered with the hike, the attraction park down Tarragona way is supposed to be the best.