I mean, not really a "Treasure" per-se, but in one box of donations found an intact, original box of Kleenex. With Kleenex still in it. It's like the guy bought it and developed some sort of lifetime immunity to runny noses...
On a side note, while the vast majority of donations are boring, china, cups, plates, etc, they not infrequently come wrapped in vintage newspapers from the day - 30's, 40's, and onward. Which keeps things interesting...