Don't take the elevator! The walk up is a fantastic part of the experience of the Sagrada Familia.
Parc Guell is rather scruffy in places and you will see guys cruising in some parts, but Gaudi's work is near the Southern Entrance.
Barcelona, wonderful as it is, is as mentioned above is very, very bad for pickpockets.
On the Ramblas your backpack will be rifled through or someone will try it on with you. So if you have a backpack, keep it on your front.
The Barrie Gothic [the old town]on the left of the Ramblas as you walk to wards the ocean is worth a look as is the seafront and on the opposite side of the Ramblas is the much poorer immigrant area, which is also interesting, but maybe not for those nervous about displaying expensive camera gear. As this is where you'll find prostitution and drugs and the attendent criminals.
There's interesting art on the beaches and Tibidabo, the Fairground above Barcelona gives you nice views of the city. Interesting church there too. The Funicula railway up to top is closed, as is funfair in October I think, but I tend to walk around places in preference to lifts or trains as you get far more opportunities for pictures that way.
It's possible to walk from the seafront to top of Tibidabo in a day. If you like walking that is!
The underground is quite handy for getting around a bit more quickly, but is a bit toasty down there, even in Oct/Nov.
Barcelona is also not so good for veggies as even the 'veggie' food tends to have meat added.
One of my favourite cities though, sea, hills and city culture all together. A rare combination.
