Make a time capsule. Take a cookie tin (or something similar), write your names on the outside somewhere and the date when it was put in the wall, then put little trinkets inside that made you think of your life around that time. Photographs of your family, maybe. You could even take a photograph of the tin inside the open wall before it's sealed up. If you have kids, have them put a favorite toy inside (preferably something that they loved in the past but no longer play with - make sure they know they are NOT getting it back). Perhaps have each member of your family write a little note about what the house looks like now and what you expect it to look like in X years.
Maybe put it in a section of wall that would be fairly easy to saw open and re-sheetrock and plan to open it up in 50 years or so. Maybe even write "time capsule" on the wall somewhere subtle (like behind a floorboard or under wallpaper). Or just put it in and forget about it and hope you or future owners come across it during renovations at some point.
Imagine that you're living in an old house and you decide to do some renovations so you break open a wall, and a canister falls out with a date of 50 years ago on it. What would be excited to find in such a container? Put that sort of stuff in your time capsule.
You might want to make sure that everything in that capsule is waterproofed. Put paper things in ziplock bags and suck all the air out before you fully seal it. You might even want to get things laminated (if that'll still let them fit in whatever sort of capsule you have. You don't know what's going to happen to that wall in the future, so try to make sure whatever you put in there will last through a number of disasters.