I've been meaning to get some proper play money for him to use, so that he can understand all those early money concepts, each coin has it's own value that stays the same, you need to use different types of coins to equal the correct amount, if you give the shop keeper too much money they'll give you some back etc etc.
Then I saw all these offcuts from the chicken coop Matt is building and I thought they'd make great play money! We got a texta and wrote on the dollar amounts and then he was off! Setting up the shop for me to go shopping! We then spent an age working out how much money we'd need to buy things at the shop, using our fingers to count on.
He does so love his numbers, my beautiful wee man!