Race at the board
Divide the class into two or three teams. One representative from each team comes to the board. You ask a question or give a problem, and the first person to write the correct answer on the board wins a point for his/her team. The catch: the students at the board only get one try. If they all miss the question, you take the answer from the first person in the audience who raises his hand. Be sure to keep this game moving to minimize wasted time from students moving to and from the board.