Thanksgiving in Prague

A friend of mine has started a brilliant tradition of taking international trips every Thanksgiving. Last year it was Vienna, the year before it was Berlin, and this year it is Prague. Work permitting, I may join her and report back. Does anyone have favorite museums to recommend, large or small, or churches with art of sculpture that blows them away? Any great outside urban art that shouldn't be missed? Smaller art/photo galleries that you like?