L and E are 30 months old.
I let the girls play with real coins this past weekend. I could tell they enjoyed the heavier feel of the metal coins instead of their plastic play money. First I gave them a mixture of water, lemon juice, and salt to wash the coins in, along with some cotton balls and paper towels for polishing them. They didn't turn out as shiny as I expected (I think vinegar/salt is supposed to create a chemical reaction in copper pennies that will get them shiny), but it did get a layer of grime off. The coin washing activity (like anything water-related) kept the girls occupied for quite a while.
After the coins got their baths, we explored some of the properties of coins. I asked the girls to sort them, but they had a lot of trouble telling the difference between the various silver-colored coins:
So we compared their sizes to see the differences: