Well, "the most interesting" is surely a subjective thing. That said, here are a few things that either I've played with, or that I'm at least aware of:
- potassium ferricyanide and mixed with ferric ammonium citrate (i.e. Cyanotypes)
- Various UV-sensitive materials are arguably "photosensitive" -- the photons may not be in the visible spectrum, but the concept is the same. A number of these materials harden when exposed, which can create interesting opportunities to make 3D "photographs".
- Infrared film(s) (Alas, many of these have stopped being produced -- but there are still plenty out there)
- Holograph film (exposed with lasers... it's an interesting process!)
- color photo paper -- exposing it WITHOUT developing it can be interesting... though you need a pretty long exposure.
- liquid photo emulsion -- this can be painted on to a wide variety of surfaces, so perhaps that's the winner, since you can choose your "most interesting" surface?
I'm sure there are numerous others, as well. Especially with exotic light sources (the "shadows" created by a nuclear explosion could likely be argued to be "photographs", right? oy! But even something more tame, like a laser or UV light.)
Also, if you can use grass, I imagine you could use human skin, similarly. And probably other biological organs/organisms (algae? etc.).