Ask HN: What would appreciate in value...

How about something that would become valuable, but is currently considered junk, eg an original IMac.

Harvest as many signed first editions of every first-time author you can find. Write them letters and hopefully get some insightful or enlightening replies. Maybe even tell them what you're planning as they may respond more favourably with an eye on posterity.

To be in with a fair shot, you'll need quite a big hole though!

You might want to seal them so that poor quality paper doesn't degrade too much though.

Are there any government bonds with greater than 30yr maturities? I'm not aware of any such US bonds. They'd mature, you or someone would get the principal in currency, and the actual bond buried in the yard would only have value as a museum piece 270yrs later.

Water. Alternatively, stocks in desalination companies.

Gold?