When we built our house, why didn't we think to put crap in the walls so someone 100 years from now will be perplexed with the mystery of the folks who lived in this house before them? And why did people put crap in their walls 100-200 years ago? Was this some sort of fad that they went through much like listening to Ipods are today? (I predict that Ipods won't last as long as the Walkman did, and we'll have something like a Brain Chip that can be loaded with the last 50 years of music and you just plug yourself to the computer whenever you need a new download.)
Anyway... junk I would have put into our walls if I had only known:
A tape of Madonna's Ray of Light (because why would I want to listen to it anymore?)
A copy of property taxes of our house during the year 1999
My leg warmers from that horrible time for me in 1984. And 1985. And 1986...
Those VHS copies of every bad aerobics' tapes I've ever owned
Printouts of Bulletin Board System conversations (this is way before Al Gore made his invention of the Internet so public)
Malibu Barbie, nekkid
Notes from high school that never did get burned
A short story started on a napkin while sitting in a King's X
And that's about it. Should be enough to make the future owners think odd things about the former owners.
And if you should catch the program If Walls Could Talk, it can be pretty interesting--for crap in the walls.