Here are a few ideas and great math sites to practice some skills that are explored in third grade at the beginning of the year.
Buying a set of flash cards and using them nightly is a great way to master the basic addition and subtraction math facts. You can also use a regular deck of cards to practice facts. Just display two cards at a time for each player in the game of WAR. Add (or Subtract) the value of the two cards to determine who wins the play. Face cards can be used if they are given a value such as zero or ten.
Remember that it takes time and LOTS of practice to master the basic math facts.