Joined by first dog Bo, Mrs. Obama read "A Visit From St. Nicholas" to children at Children's National Medical Center in Washington.
One child asked what she plans to give the president. Pointing to news cameras, Mrs. Obama said if she says anything, "they're going to print it into the papers tomorrow."
One boy recommended an iPad or an iPad mini. The first lady promised "it will be something nice."
Mrs. Obama did not address Friday's deadly shooting at a Connecticut elementary school and spoke shortly before her husband addressed the nation.
Her visit was part of a tradition by first ladies dating back to Bess Truman.