Do not drink sangria. Drink red wine, especially rioja. Rioja reserva or gran reserva (older one). It’s a great wine under reasonable price. You can also buy it at supermarkets, like Lidl, where you can buy Campo Viejo Rioja. If you are more of a beer type, try San Miguel or Estrella beer.
Buy tickets for Park Guell in advance, because it’s very crowded.
Buy T-10 metro card (valid for 10 rides), it’s transferable and most economic. The price is 10,20 € and the price of a single ticket is 2,20 € (excluding one to/from airport).
Have a meal at one of a numerous restaurants at Barri Gotic.
When entering a pastry shop/bakery take a notice if there’s a machine issuing a ticket with your number.