Sue...I've been to Barcelona several times - there is a bus that circles the sites and I think you can catch at the end of Las Rambles - near the Columbus Statue. I haven't done that, but I've hear good reports.

Link for bus information

I have used a driver on two occasions and always asked for particular sites - that takes some research on your part, but they usually recommend things that are interesting if you are open to that.

I wanted to see all the Gaudi sites and they are outstanding, but there is much more to Barcelona than one architect!

If you're interested in Art there is a wonderful Miro museum as well as a museum devoted to early works by Picasso.

One of my favorite guidebooks for almost any area would be the Eyewitness Guides. They publish a volume for Barcelona. It's a good purchase for planning your visit, or just taking an armchair tour of Barcelona.