I like the idea of a forest, or a mental asylum. For the former, I'm actually planning a screenplay for a short film to shoot with my fellow film-makers/friends at university, which isn't strictly a horror but is meant to creep the viewer out. Basically, it's a love square type thing, an emotional drama, but one of the characters casts a spell from some 'black magic' website she found (said character is interested in the occult and whatnot, while her friends are skeptic), and it's never explained if the spell was real or not, but various weird stuff starts to happen. The idea is that the background music starts fairly normally but becomes (very subtly and gradually) much more tense and creepy, there will be deliberate continuity errors between cuts to try and heighten the tension, and so on. And it's set in the middle of the woods, on a camping trip (really Roundhay Park, and 'folly' in the shape of a castle, in Leeds).
As for the other, I can probably come up with a decent horror tale set in a mental asylum, but without following all the old cliches. Oh, and yes, for the record, I did love Shutter Island and thought it was an exceptionally excellent movie.