This thread got me thinking about items I have left for future discovery.
I was living in Queens, NY, when Reggie Jackson retired and I happened to be sheetrocking a bathroom on that day and put a copy of the Daily News inside the wall.
Other times I have left a coin inside walls.
Most recently, while remodeling my own 100 year old home, I left a plastic container in a void above a fireplace. Included was that day's copy of the KC Star, the front page of the NY Times (printed from online), some contemporary coins, a tile from the original hearth of the fireplace, and a disc of photos of the house before and during renovation along with some pics of me and my wife and the dog.
The only thing I've found is an old gin bottle stuck in the joists in the basement. My plumber tells me that is really common and he has a large collection of those.