If you need to pick up some cheap clothing from the usual kinds of stores, Portal Del Ángel is not a bad bet. There are no surprises here (think H&M, Zara, etc. etc.), but for convenience, it's a great place to go.
The pedestrianized street makes for a good stroll, despite the masses of people. On weekends, you'll find stalls and boots at the part closest to Plaza Catalunya, selling locals crafts and delicacies. Take the street all the way till the end, and you'll get down to the Cathedral for a bit of sightseeing.
The chain stores are fairly boring, but looking down the side streets reveal a number of different specialty stores.
Mon-Sa: 10:00-20:30