divide students into two teams and then, either show a flash card or say a problem. The students (about 2-3 students per number) make the answer on the floor with their bodies. If the answer was 35, 2-3 students make the 3 and 2-3 students make the 5. The team that makes the right number the fastest and correct wins.
(To make sure that all students get to participate, no student can help make the answer twice in a row.)