Sum-o (which is a take-off on the word “sum,” in case you didn’t notice) is an addition practice game which provides young children with the opportunity to investigate different strategies to optimize their play. Like chess, it involves practicing addition and subtraction skills among the two players, and requires them to anticipate the future moves of their opponent. At the same time, there are definite outcomes that can be anticipated in the easier games which grow more complex as the numbers get larger.
I used this game with my kindergarten through 3rd graders and they really enjoyed the strategic thinking that had to take place in order to optimize their play. They also liked the fat that there were two ways to win: either by hitting the "target" number or forcing their opponent to go past the "ring" number.
This comes complete with different versions of games, including a "Jr." version for kindergarten and 1st graders, and the standard version for 2nd and 3rd graders. There's also a discussion of strategic play and different variations of the game.