If you're making a day trip in the direction of France (the area around Girona), visit the Salvador Dali Museum, and also his wife's museum, the Gala Dali Museum. Fun and campy.

As for gay, lowbrow food, go to Mar Bella, the officially sanctioned nude beach in BCN, which is as gay as it is straight. Don't confuse it with the other gay beach near the new W hotel. Have a beer and some fried potatoes while enjoying the scenery. The snack bars might be closed for the season by mid October. I'm not sure.

Speaking of the W (the dough-blay-ooh-vay in Spanish), pop into the lounge near the top of the building. Not gay, but a fun thing to do.

Check out the cemetery in Poble Nou. Walk to the oldest part and look around. Cool and a bit creepy.

If you're into design, check out Mies van der Rohe's Barcelona Pavilion. http://en.wikipedia.org/wiki/Barcelona_Pavilion
There's also an interesting museum across the street. I think it's called the Fundacio la Caixa. There's another Fundacio la Caixa near the Forum too I think.

If you find yourself in the area of the Forum, pop into the Diagonal Mar shopping center. On the supermarket floor of Alcampo is the world's largest wall of ham. And lots of weird Spanish products we've never seen here.

Not gay, but I have two favorite restaurants in the city. El Glop, which is in Gracia is one. The other is Tapas24, which is just off Paseo de Gracia on Diputacio.