If you want a posh dinner while you’re in barca, I can’t recommend highly enough a restaurant called Pla; http://www.elpla.cat/pla/barcelona-restaurant/index.html – was good service & great food & wine when we went there. (we got engaged there!)
Portions aren’t big, price wasn’t too bad but the food was great. possibly the nicest meal we’ve had over the last 5years.
We walked from the Sagrada Familia & the beach, then back to the top or Las Ramblas – not a good idea, was a long trek although we probably didn’t take the most direct route!
Definitely a good idea using one of the circular tourist bus routes to get around as it’s quite spread out – not normally a fan of them but in Barcelona it seemed a good way to find where things are & decide if you want to go to them as things are spread out.
This website: http://www.barcelona-tourist-guide.com/ was the most inspirational for me.
We stayed at Hotel Jazz, and got breakfasts at local cafes, it was ok.