Write a selection of numbers on the board, which could be the answers to a particular topic you are studying or are trying to reinforce, e.g. adding two-digit numbers.
25, 28, 84, 39, 14, 19
Ask two children to come up to the board and give them a pen each.
Now, ask them a question, making sure that the answer is one of those shown on the board (e.g. 19).
The children should work out the answer to the question as quickly as possible and draw a ring around the correct answer. The first person to identify the right answer wins. Repeat with other children.
This could be used with numbers / fractions / shapes, or topics within other subject areas.