Well, I've found most of the usual detritus ... paper clips, playing cards, piano lesson assignments, combs, hair brushes, meat rationing points from WW II, legos and other plastic toys. Oh yes, and guitar picks, pencils and the usual items in the ongoing battle between piano technicians and church musicians.
Also occasional coins anywhere from a penny to over 50 cents. (If the customer is around, i usually hand it to them with a comment about, "Here's your downpayment for the work" or similar.
But as far as the term "stash" is concerned here is what I have either found directly or know about from another technician I spoke directly with.
* A stack of Playboy magazines
* A chocolate candy bar (like Snickers or similar)
* My father found some silver dollars
* A colleague found $20,000 in folded-up 20, 50 and 100 dollar bills. Of course he showed the current owner of the piano (who was not the one who hid the stash). Oddly enough he never got even one of the 20-dollar bills as a thank-you.
Then, there's the animal-based stashes...
*All three sections under the keys of a console filled with dog kibbles
*Bright, shiny things
It must have been of human origin, but I also found a cup of sugar (just the sugar, not the cup) by the treble key end block of a Kimball Whitney spinet.