My husband and I will be taking a cruise this fall that starts in Barcelona. Neither of us have been to Spain and would love to come in a few days early, but because we’ll be away from our kids for too long already, we will probably only have one day to see Barcelona.
The Lonely Planet website calls it the hippest city in Spain. Can the Dopers give us some good ideas about the “don’t miss it” stuff as well as maybe some “off the beaten track” ideas?
Thanks!
You have to go see the Sagrada Familia. It is a cathedral designed by Gaudi. It is still under construction, and has been for a long time. There is a lot of cool stuff to see and do there. Going out at night is truly amazing since Barcelona doesn’t seem to sleep.
Also by Gaudi is the wonderful Park Güell (great for a picnic) and the striking building La Pedrera. The public transit is fairly good, with an excellent metro line. At night, walk along the wharf where all the vendors are selling and there are a ton of resturants. Have some paella and watch the boats sail by.
Off the beaten path: There’s a rather good military museum in an old fort by the ocean (forgot the name). It takes a skyride to get there so that also allows for great views of the city.
Even farther afield, there’s an old monastery at Montserrat–a quick train ride out of the city. It’s an excellent trip, with great views and exploring, although with only one day, I’d still recommend taking in as much of Barcelona as you can (it’s a very good walking city, but there are also tour buses that show you the highlights)
Ooh, can I come with you? Please?
It’s a great walking city; you’ve definitely got to take a stroll down the Passeig de Gracia (home of various Gaudi buildings, including La Pedrera) and Las Ramblas (for street performers and sheer weirdness). Also, the Picasso and Miro museums are worth a visit. You can’t go wrong with nightlife, as brujo says, although after six years I’m hesitant to recommend any place in particular.
Watch out for pickpockets and purse snatchers, especially in heavily touristed areas; keeping your valuables under your clothes is definitely a good idea.
Could you send back to me some Spanish Boots of Spanish Leather? 
I agree with brujo, you must go see the Sagrada Familia - you can easily spend 3-4 hours there. Both the front and the back are covered with amazing, intricate statues and carvings (the back is in indescribably complex), and you can climb up the spires almost to the top (it’s massive, too). Just don’t do what my friend and I did - we spent hours there and taped the entire thing, then accidently taped over it. Why? Because my friend had seen a Pizza Hut from the top and we rewound the tape to try and find it (we were damn hungry after all that climbing! and lonely for American food after two weeks in Europe) and forgot to reset it. 
Sacrada Familia is a must. To me it’s a church I would attend only in a psychotic nightmare. Still, it is undeniably beatiful and amazingly original.
If you like Picasso’s stuff at all, definitely go to the Picasso museum, they’ve several rooms of his versions of ‘Las Meninas’ by Valazquez, as well as a gallery of his very early drawings (detailed anatomy drawings done at at 8 or some disgustingly prodigic age). Useful for dispelling the idea that Picasso painted that way cause he couldn’t make things look ‘real’.
Walk along Las Ramblas, see the flowers and the endless street performers (doing the same tired crap they do in every city). Get some seafood, I had the best calamari I’ve ever had in Barcelona.
The olympic park is kinda cool. It was eerily empty when I went there, reminds you of how useful all those millions of dollars really are after the sixteen days are over.
Anyway, good luck. You almost can’t go wrong, Barcelona is a fun city. But do be careful after dark, you’ll not run into much serious violent crime, but there are a lot of pickpockets, and petty thieves. Especially around Las Ramblas and near centers of mass transit.
Thanks, this is exactly what we need! Some of the best information can only come from experience, not a guidebook!