Input Output
In this game, each player makes a T-chart on paper, labeling the left side of the chart as Input and the right side as Output. Players then make up a rule, such as Input + 4 x 2 = Output, and fill out both sides of their chart according to that rule. When the chats are completed, exchange them with one another and try to figure out what rule the other players used to complete their charts.
Target Number
Using a standard deck of cards, deal each player five cards and then leave one card face up in the middle of the table. The card in the middle is the "target" card and players must apply any mathematical operation they can think of to the cards in their hand in order to achieve the value on the "target" card. Points are awarded each round to players who are able to make the value on the "target" card.
Math War
Using a standard deck of cards, deal all the cards face down evenly between two players. Each player then flips over a card and the first player to come up with the correct answer for the sum of the two cards takes them both. The game ends once one player has all the cards. You can even change up the rules to incorporate subtraction, multiplication or division.