I spent a week there last week. (Left 8/22).
Book Sagrada Familia ahead of time for the skip the line. WELL worth it. Be sure not to miss the museum in the basement - you have to exit on the Passion side, loop around to the left, and it's underneath... Lots of Gaudi's design strategy, scale models, how they made the plaster, etc.
Book Picasso museum ahead of time for the skip the line. That must be printed out - so book it when you have access to a printer. Or - if you're an idiot like me... there is an internet cafe BEHIND the Carrefour on Las Ramblas (at the Plaza Catalunya side). Go through the Carrefour to the back entrance - go out, turn right, two doors down. They can print out the tickets for about .15 euros per sheet.
I agree with walking Las Ramblas but don't necessarily agree with avoiding the night. It's crowded and full of people/kids/old folks until 11pm or so... Late at night can be sketchy. I fully agree with the idea to watch for pickpockets... keep your money close and be careful with purses/backpacks. It's crowded.
La Boqueria is cool - but not a bargain and we found the quality of the food only so-so. That said - there's a place that serves "to go" food on the outer edge - organic falafel, paella, etc... reasonable prices. Perhaps we were spoiled having been in Nice with the Cours Saleya market, and the wonderful markets in Aix-en-Provence, the previous weeks. For produce, the Carrefour was actually better quality and WAY better prices. (I'm still grumpy about the guy at La Bouqueria that swapped tomatoes on me - I'd picked 2 nice tomatos and he did a slight of hand to give me old, starting to rot, tomatoes)
The beaches in Barcelona are awesome. The ones a bit further north than Barcelonetta are less crowded than Barcelonetta.
Don't limit yourself to the metro. With google maps - it's easy to figure out buses - which may take you a lot closer.
You can buy a 10 ride ticket for the metro/bus. It's 9.95 euros. The metro pass assumes you ride at least 3-4 rides a day to break even... We walked a lot so the 10 ride ticket worked out as a better bargain. For the beaches the bus took us much closer. The ticket gets inserted into a machine at the entry to the metro or ON the bus. Easy to use.
Since we're not late night people - we tended to do tapas for dinner. Some of the places use a "toothpick" system of accounting. They have a bar/buffet of tapas... you take the plates you want... and save the toothpicks in a cup on your table. When it's time to settle up, the server counts the toothpicks and charges you accordingly.