It is a great idea. There will be parties everywhere. Something you need to know about new year in Spain is this really fun tradition. When the new year comes, people gather to hear the bells. They strike 12 times (the first 4 don't count because they are the quarters), and with each strike, you have to eat one grape. This tradition means you will enter the new year with good luck if you finish all of them in time (everybody has a full mouth and juice all over their faces and they hug and laugh).
If you are still there the 5th at night, try to catch one of the "cavalgatas" where there are people dressed up as the three kings, their servants, and even real camels sometimes. They march down the streets and throw candy to the people that come out. about 8 years ago
barcelona is great, there are many things to see. Sagrada Familia, Park Guel, and other Gaudi creations are very interesting. Barcelona inspired alot of famous artists such as Picasso, Miro, Dali, so there are plenty of art galleries. Catalans are always having festivals. Also you can do a Bike Tour, they are a really great way to see the city and sites. I have done Fat Tire tours a few times, that is always fun. There's also a lot of really great restaurants, cafes and bars. about 8 years ago