Everything is closed on Sundays. EVERYTHING. Well everything except for Chinese owned stores/restaurants and tourist spots (on the Rambla).
If you're looking for shoes and street wears, check the "el born" area (between the Rambla and the MACBA (museum/skate spot). Maybe some in the Gothic quarter (opposite side of Rambla).
5th Ave designs and big name retailers are all on the Passeig de Gracia.
Also check out the Montana shop (paint/markers/books/some tees). Check google for the address, it's like one or two metro stops from Catayluna. Markers are cheap, since it's all made in Barcelona.
Barcelona is TINY, you could walk a loop around the city in a "hour." If you get lost, you'll probably end up walking into an area your familiar with and wonder "wtf, how did I get here?"
Also be on the look out for Neckface tags everywhere. Barcelona is like his 2/3/4/5th home.