A ton of money...
About 400 pennies have a mass of 1 kilogram, which means that 400,000 pennies have a mass of 1 metric ton.
About 800 jumbo paperclips have a mass of 1 kilogram, which means that 800,000 paperclips have a mass of 1 metric ton.
And a dollar bill has a mass of 1 gram (pretty much perfectly), so a kilogram of dollar bills is $1000, and a ton of money will make you a millionaire.
This was a much harder project for the sixth graders than I was expecting.
(The next question, of course, which I think I'll have to save for a half-day or something, is How much space would you need for all these pennies, paperclips, and dollar bills?).