Best tip I heard for la Sagrada Familia is to get there early to avoid the crowds.
As far as restaurants are concerned, there are so many and the city is so massive, that if you try to chase after someone's recommendation, you will be zipping all over the place and passing loads of other great restaurants while you do it, so I don't bother with that, unless its a really unique restaurant such as Tickets.
Sadly we didn't go there because DP isn't very adventurous food wise
. Often, I just go for the nearest Spanish restaurant I see, and they're usually pretty good.
Just don't go for the giant glasses of Sangria on Las Ramblas unless you are happy with the equally giant price (I think they were about 12 euros each and we had to go to the cash machine to pay the bill).
There's also an Arc de Triomphe (like in Paris) at the top of a really nice park and I loved the outdoor escalators near Guell park.
There's a lovely fountain that is lit with with a musical accompaniment at night. I think its near Montjuic/Olympic Park (which has great views in the daytime).
If you're going for more than a couple of days, you can also get the train to other nearby seaside resorts for a change of scene.