i do this all the time ... and have a whole drawer FULL of photographs that have disappeared .
what i do is a cross between a solargraph, as newt-on-swings mentions ... and a lumen print
you just need a box camera, or you can even use your 35mm camera ( but the image will be as big as a 35mm negaitve )
you take a some photo paper and put it in your camera ( this can be done in daylight, it doesn't really matter )
then, you leave your camera bulb or time &c ... it takes between 45mins and a few days .. and all depends ...
when you remove the paper, there is an image on it ...
it will be there for a little while, but slowly it will disappear ( even in darkness ) - the paper turns grey, and the image just vanishes ...
impossible to fix in a conventional way( if you try, the paper will turn as white as it was in the box.) i've been making cameras
to do this sort of thing for a little over a year some small ( 2x3 ) some large ( 11x14+ ) i have one i am finishing today ( or tomrrow ) for someone
who ordered one from me ...
i mentioned lumen prints ... you can also do something sort of like that, but without plant/biological matter you are contact printing.
you take you "negative" ( maybe a piece of acetate with a drawing on it, maybe a large format negative, maybe keys an change ( photogram )
and you put them on a regular piece of photo paper, and leave it out in the sun ... you will get an image ( positive image if you used a negative )
from whatever was blocking the sunlight ... and again, it will disappear eventually ...
if you are interested i can make a camera for you ... glass lens and all !
have fun !