Yes I was going to suggest an unfixed print, though I do not know how fast it will fade. You will want to put it in a black envelope though, so it doesn't fade (go dark) before opened.

The other idea is to make some kind of peel apart thing like a polaroid that reveals the image when you peel it apart, but exposes it to the chemistry that will destroy it.

(such as an unfixed print being exposed to a developer (turns black) or a bleach possibly (turns white)).