A dollar bill weighs just about a gram.
If you've got an ounce (~25g) halve it, halve it, halve it, third it (by eye). That'll give you about a gram, tell 'em to take it or leave it. L
. One cc of water weighs a gram . Wikipedia claims: 0.99820 g/cm3 at 20 °C
Make an evenly-balanced see-saw. Hang a known weight on one end, then move the balance point so that the known weight would be balanced a a single gramme on the other side. For instance, if your known weight is a 25¢ coin, weight 5.67g, you place it 1cm from the fulcrum (pivot), and place your mystery weight 5.67cm from the other side of the fulcrum - if they balance, the mystery weight is 1g.