Sagrada Familia is an eyesore. But Google it and see if it's your cup of tea Littlefluffy .
I'd second La Boqueria market for food, it is about halfway down the Ramblas. About level with it, on the opposite side, is Cafe L'Opera which is lovely.
Parc Guell is beautiful, but they have stared charging to see the famous mosaic lizzard and the wobbly benches (I'm sure there is a proper name! ), giving you a set time, too. I tjimk it is worth it.
As far as I recall, all the tour buses pass through the main square Placa Catalunya, at the top of the Ramblas. You get great views of everything up there.
Bars come and bars go, so can't advise on nightlife, although Razzmatazz was always great (google to see if they've got anything on).
A great tapas restaurant, proving it is still there, is Cerveceria Catalana: Rambla Catalunya (which runs parallel with Paseo de Geacia, and starts from Placa Catalunya), go up about 5 blocks from Pl Cat and turn left onto... I want to say Mallorca Street. I'll search it. It is right on that corner. Really really nice good, full of locals.
If anything else pops into my head, I'll post it 