My 8yo son received an inexpensive card game, Sleeping Queens, for his birthday. He'd play it every day if we would. (His 4yo brother is also able to play with adult supervision.)
The object of the game is to collect queen cards or points. Various character cards--king, knight, dragon, sleeping potion, wand, jester--cause a variety of actions. Although the game feels like pure entertainment, it includes a mathematical component; you can discard (a good thing!) three or more number cards that make an addition equation. For example, if you have cards numbered 2, 3, and 5, you can discard them if you notice that 2+3=5.
As a fascinating side note, the game was invented by a 6yo girl who lay awake one night, dreaming up the rules for the game.
The box says for ages 8 and up, 2-5 players, about 20 minutes/game.
LilDude says he highly recommends it! It would make a great math workbox.