My favorite things to do in Barcelona included sipping great café con leche in the morning at a local café, walking around the old town and the Eixample, enjoying a long, leisurely lunch, seeing some of the famous art and architecture, and dining on a late supper of seafood tapas before a long night of partying. There are tons of things to see and do here, including having a look at the famous Gaudi buildings, walking through the beautiful parks and along the beach in Barceloneta, visiting the Miro museum, and relaxing in the old town. There's also plenty of shopping to be done in the Eixample - the area north of Plaza Catalunya, where there are a number of nice-looking restaurants and bars as well. Finally, eating and drinking in Barcelona is a fabulous experience - Spanish cuisine with lots of great seafood washed down with inexpensive but delicious Spanish wine.
For some great tips on sights, food, nightlife and lodging, check out Time Out Barcelona.
Quick Tips:
While there are many things to see, Barcelona is best enjoyed at a leisurely pace - if you try to do too much, you'll definitely lose some of the enjoyment. This happened to me one day, as we went from La Sagrada Familia, up to Mont Juic and across to Barceloneta in one afternoon. While all of these places were lovely, we didn't have nearly enough energy that night to properly enjoy the great nightlife Barcelona has to offer. If you only have a couple of days in Barcelona, I recommend seeing the old town and walking down to Barceloneta (to the beach) one day, and another day in the Eixample seeing the Gaudi buildings (La Pedrera, Sagrada Familia, etc.) Next on the list could be a walk (or bus ride) up Mont Juic to see the magic fountain (turned off when I was there.) and the Fundacion Miro (Miro museum.) I found the cable car ride from Mont Juic to Barceloneta to be a rip-off. Strangely, Barcelona is not so impressive from above. I also found the Olympic village to be disappointing - okay if you're an Olympics fan, but not very Spanish in my opinion.
Best Way To Get Around:
If you don't try to do too much, and your hotel is conveniently located, you can pretty much walk all over the city. Sometimes, however, you may find yourself quite far from where you started, so you may want to use the bus/subway system to get home. Since my hotel was near Plaza Catalunya, I wrote down the numbers of all the buses that stopped there, so when I found myself far from home, I knew which buses would take me there (you can also purchase a bus map, if that's easier.) The subway system was also useful in getting to/from outlying areas or across town.
Getting from the airport was a breeze - there's a train that runs directly from the airport to Plaza Catalunya.