How about creating a solid silver (or gold) cube that consists of precisely the amount of material 1 BTC could buy at a given time, let's say at one block in the blockchain.
You create the block, engrave the block number somewhere and sell it for 1 BTC + X (X covering your costs and some profit).
Let's say you do this once every 144 blocks, i.e. once per day.
Every single block will have a different size representing the spot price of silver relative to Bitcoins exchange rate at the "block time" of its creation.
I guess this could become a nice collectible.
Not sure if I'd buy one, though
![Roll Eyes](https://bitcointalk.org/Smileys/default/rolleyes.gif)
That's a really good idea but instead of either gold or silver could you make them in platinum ? Heavier metal so smaller cube and nobody else is using this material.