Unlike most cities on South America's west coast, Buenos Aires has fairly consistent rainfall all year long, and you might see a bit of the wet stuff even during December. Still, the temperatures should be ideal for being outdoors, and this is really mostly an indoor city anyway, so when it does rain you'll be able to pop into a shop or museum or cafe and have a great time.
I’m sure you’ll have a great time. I don’t have any good suggestions for where to celebrate Christmas, but I’m sure you’ll have no problems finding something by just walking around or checking online. The top draw in Barcelona is the architecture, and especially the 20th Century buildings by Gaudi. I recommend the hop-on, hop-off bus tour because it allows you to see almost all of the famous buildings from the street in just a few hours. Park Guell is worth a visit, but of course the main attraction is the Sagrada Familia cathedral. Check opening times and reserve a ticket in advance if you can.
The 17th-annual Hilton Head Island Motoring Festival and Concourse d’Elegance takes place Oct. 26-Nov. 4, attracting automobile enthusiasts and all who enjoy the southern charm, splendid fall weather, and extraordinary classic cars. The Hilton Head Island Oyster Festival (Nov. 9-10) is an oyster extravaganza with live music, crafts, a Kid Zone, and sports lounge. A stay at the Inn and Club at Harbour Town at the Sea Pines Resort will provide luxury and comfort in low country style. Or book a home rental through Vacation Company.
Traveling during the last ten days of December means crowds and exorbitant prices throughout much of the world, but the first two-thirds of the month—before holiday airfares and hotel rates go into effect—can be a serene, value-laden, and just plain lovely time to vacation. Europe is all decked out for the holidays—with locals generally friendly and in high spirits; in the Southern Hemisphere, spring is in bloom; and in the Caribbean, Hawaii, and Mexico, hurricane season is over, the weather is gorgeous, and there are free upgrades galore.