Larry Niven and Jerry Pournelle suggested using spent fuel rods. The benefits are as follows:
Nobody would hoard the coins in an effort to avoid critical mass
Pending bank robbers would be easy to spot due the the lead suit
Payments for international debt could be delivered by ICBM
I can't remember the others.
If you lose your coins you can find them with a Geiger counter, or wait till night time and they will glow in the dark.............