Here's a Hypothetical

It's silly, I know. 

You have a time machine.  But it can only take you back to 1820.  200 years.  You can take 6 $20 gold pieces with you and what you are wearing.  After a short time you come back to the present, a week or so.  So don't kill your own 5 greats great-grandpa while you are back there.

If you invest that $120 at 7% you'd get $90,000,000 accrued in that time.  But how to arrange that principle to sit untouched until adult you in 2020 can come around and collect? 

You could bury the gold pieces in ground you know will be undisturbed for that time.  I can think of a bunch of places nearby I could pull that off, but what does that gain me?

I can buy something rare for $120, and bury THAT.  Something that people today would pay a lot for.  But what?  What object could I get that could survive being entombed that long?  A set of Washington's dentures?  You wouldn't get close to 90 million for them, but something.  His saddle pistols fetched 2 million.  I wonder if the Washington family would part with them for $120?  If they still had them 20 years after his death.  I doubt they'd be available.

11 year old Lincoln's school book?   Good luck proving the provenance of all this. 

Buy a $120 Faberge Egg?  Impossible.  The first one won't be crafted for 75 years, but you get the idea.  An object d'art.