Check out this site on Math games: http://www.coolmath-games.com/
It has a lot of free Math games to help you work on your arithmetic skills.
A recommended game is “Count Cubes”: http://www.coolmath-games.com/0-countcubes/index.html
This game really helps in visualisation skills, further more Counting Cubes is a legitimate question that can come out in PSLE.
Hope you have fun trying out the game.