My first thought was to collect some nice shinny new pennies for smashing. I soon found out that Pennies made after 1982 have zinc in them and that after a while the zinc turns black and smears. I was surprised at how easy it was to find pre 1982 pennies. I started out shining them with Brasso, but then I decided I like the look of the older penny because after smshing them, you can often faintly see the penny design on the back. I carry mine in an old film canister.
Have fun collecting them. It can get addicting.
Chessie