I like the dollar coin idea, although I remember when my uncle gave my brothers and I all five Kennedy dollars and told us to save them because they'd be worth something some day... Wish I still had those. They're worth less than they were then, but they sure were cool!
If my kid found $50 under his pillow the first I'd hear about it would be, "Hey dad, check out the cool ________ I got with my money from the tooth sissy!"
J-