In my opinion, it could not be something that is a commodity like gold or silver.  That stuff will probably not be worth much more, taking inflation into account, than it is now.

To me, what you need to get is something that would be seriously collectible.  I mean, imagine if you had a copy of the first edition of some famous book from 1810 right now.  It would be worth a lot because of the fact that it is collectible.

So what you have to do is predict what would still be famous in 200 years.  Maybe you should get some Obama stuff since he's the first black president.  Or maybe you should buy an iPad.  People might be really interested in years from now to see what we thought was all futuristic.  Otherwise, try to predict what books or movies will still be famous and buy copies of those and preserve them.