Though I haven't been there yet, I would recommend a day trip to Cesky Krumlov.
For the most part, you can cross the entire Czech Republic in about 4 hours, so that makes the entire country accessible. Domestic travel by train and bus is extremely cheap, too.
Last year during this time frame, there was snow on the ground in a lot of the country, so be prepared! In Prague, it snowed just before Christmas but didn't stick.
Getting a little more ambitious, you could try to go to Nurnberg in Germany. It is reputed to have one of the best Christmas markets in Europe. It will likely take about 4 hours to get there, so you may want to consider an overnight.