I have always done the same on all property renovations. A tupperware box, usually with a small Teddybear, Some photos, with names of the family, and a note telling the finder a bit about us.
I also put our first Dogs collar and tag, around the trunk of a Silver Birch tree that we planted, to remember him. When we sold, the leather had been enveloped into the growth of the trunk, with just the name tag sticking out. The people who we sold the house to, who remained friends, told us that the name tag recently got enveloped by the trunk. They have not done any major alterations. So the tupperware box is still inside One of the stud walls.
We also built in secret, letterbox sized connections, between the Two of the bedrooms. There children had both these rooms, and we were told they were delighted when the found the secret connection between the rooms. It took them over Two years before they discovered them.
We told them about the box in the wall, but have refused to tell them where it is.