I live outside Barcelona and, although I don't go into town a lot at night and I'm not that much into the "scene" there, I'll try to give you a few tips, for what it's worth.
As far as bars, clubs, discos etc. are concerned, you can find trannies pretty much anywhere: I've seen some in the Pacha and other clubs. Otherwise you can have a look around the so-called "Gayxample" district, which is situated in the streets on the left-hand side of Passeig de Gràcia (Paseo de Gracia*) as yo go up it from Gran Vía de les Corts Catalanes (Gran Vía de las Cortes Catalanas*). This district covers the following streets: Carrer de la Diputació (Calle Diputación*), Consell de Cent (Consejo de Ciento*) and Carrer d'Aragó (Calle Aragón*), and extends along them until they all intersect with Carrer Comte d'Urgell (Calle Conde de Úrgel*).
As for street action, I believe that there is some towards the Camp Nou football stadium, and also in the Ramblas. However my advice is to give street action a miss: you can get hassle from the police in the Camp Nou district, and the Ramblas is a definite no-go area at night: lots of rough trade, pickpockets, and other such types; indeed I even give it a miss during the day, because it's not the finest part of town and even by day is full of pickpockets, con-artists, grotty souvenir shops, rip-off bars and clueless tourists.
* You'll notice that I give the Spanish street names in brackets, because since Catalan and Spanish are both official languages here, depending on who you ask your way from, it's a good idea to have both versions handy.