What's the best thing to do in Barcelona,Spain if your only there for one day?

The best is to walk along "Paseo de Gracia", a beautiful avenue full of buildings with catalan modernism, that is World Heritage by the UNESCO and are stunning. The Most famous is "La Pedrera" also called "Casa Milá". At the end of the avenue, there is the centric square "Plaza de Cataluña" and next, the world famous "Ramblas" street, where are in a popular nightlife district, full of shops, restaurants, street artists, street vendors... Very cosmopolitan. The Old Town "Barrio Gótico" is there.
Nevertheless, the symbol of Barcelona is also a catalan modernist building, the cathedral of "La Sagrada Familia".
But it would be a pity if you miss the Harbour and the beach, but there are a lot of things to do in Barcelona!!!
If you have little time, I recommend you to see Paseo de Gracia, Plaza Cataluña, Las Ramblas and La Sagrada Familia.

What Salva said. Only if you want to spring for it, the tourist bus of the Eixample (Passeig de Gracia - about 20€) is a good way to see more and save your feet. I don't generally do those touristy things, but Eixample is pretty big, and it worked to take it all in that way.

Both the above offer excellent, sound advice except Salva unintentionally misleads a little by referring to the Sagrada Familia CATHEDRAL straight after telling you about the Barrio Gótico (Barri Gotic in Catalán).

The 14th century Cathedral IS in the Barrio Gótico, but the ultra moderrnist Templo Expropiatorio de la Sagrada Familia is a couple of miles away, and is a 'must see'!

Since you have such a short time in the city the Metro is handy for getting round; make sure you're on the correct colour-coded line in the right direction and you'll be fine.

There are a number of discounted tickets available, including (I think) one offerring one day unlimited travel, which will save a lot of money. This link should help...

http://www.tmb.net/en_US/home.jsp