The German city of Dresden on the Elbe river is located just 130 km / 80 mi Northwest of Prague and therefore it's a good destination for a day trip. This city in Saxony was badly damaged during WWII in 1945 but it rose like a phoenix from the ashes. The renovated and rebuilded landmarks of Dresden include the famous Frauenkirche (Church of Our Lady), Cathedral of Holy Trinity, Zwinger palace, Semper opera and more in the Old Town. In the New Town there's a lot of contemporary art to discover including some very interesting architectures... During the month of December, Dresden is popular for its Christmas markets.
Very nearby are the Elbe Sandstone highlands (also known as the Saxon-Bohemian Switzerland). This superb gem is a national park on both sides of the Czech-German borders and it's a great stop on the way from/to the Dresden. Its views of the Elbe river valley and the Bastei bridge are very unique and definitely a nice addition to the Dresden trip.
Daytrip from Prague - 9-11 hours (including 4,5 hours drive)
5.000 CZK / 230 USD / 200 EUR for the guided service + cca 7.500 CZK / 345 USD / 300 EUR for transportation costs
Please note, as Dresden is in Germany, it's necessary to have with you your passport and Euros.