Listen to counting rhymes and songs. "Three little monkeys jumping on the bed; one fell off and bumped his head. Mama called the doctor and the doctor said, 'No more monkeys jumping on the bed!' Two little monkeys jumping on the bed..." Any variation on this counting rhyme introduces basic subtraction. Look for children's games and music activity tapes such as the Wee Sing series that features songs about numbers.
Make a recipe with your child. Give your child the measuring cups and bowls and let him measure out the ingredients while you read the directions out loud. An easy — and delicious — way to introduce concepts such as volume and weight.