A handful of coins
This is a great fall-back for when you find yourself waiting somewhere with your kids and you're empty handed. Take one of each kind of coin from your wallet and play guessing games with them. For my youngest child, I give clues to describe which coin I'm thinking of: "This one is silver, it has a face on one side, it's the biggest" etc. For my older children I make it harder, "This one equals twenty-five cents" or "100 of these equals a dollar." It's a great way to learn the value of different coins.