Several years ago I went and mixed up some quick set cement and filled a shoe box with it and stuck some cash in a plastic container in the middle of the cement , wrapped it all up as a Christmas present for nephew. 
YOU HAVE SAVED MY LIFE Thundley.
Every few years Hubby and I get a wild hair and make something outrageous for my eldest son and wife. This son makes much more money then we do and can afford to buy any darn thing he wants. We have taken wood from old apple trees that died of old age on the family home stead and made clocks, one year we made a paper mache light house that was most likely thrown out the next day. Lots of crazy stuff I don't really remember.
This year I was stumped to the max as to if I should make lamps out of driftwood or good old money. Darn Thunderely, you are brilliant, what a way to get back at the kids for all those diaper changes.
I wonder if I should line the cardboard box with a plastic bag and just pour in Quick Set concrete, let it set open to the air.?
There are a world of possibility's here, those gold one dollar coins say $50.00 layered in the brick,
A nice broach in a box wrapped in plastic smack dab in the middle of the brick.
If you have any more brain storm ideas on a really fun project, please let me know.
Bless you Thunderly, my heart felt thank YOU.