A standard roleplaying environment: the tavern. This is another easy one to create in your own home, providing you have a large enough group of roleplaying friends. Let us assume your PC group is no larger than five, and that the rest of your friends can be NPCs. Let us also aim for an exceptionally seedy and unscrupulous city tavern.
In a good LARP it is desirable to separate the players out individually at some stage, because they are more suggestible and afraid when they are alone. Some form of individual challenge is probably the best way of doing this. One possible setting for this is a cave system, and it is probably one of the easiest to recreate realistically, and one of the most perfect for playing on fears of claustrophobia and darkness.
Foot Coral. It attatches to a foot like moss, almost gluing itself to an organism. It gets its food supply from the ground, and does not leech off the host in any way. It is not a malignant organism, and is entirely harmless. But it only grows on human feet, and even then, never on the left foot. An organism that it grows on can feel touch through the coral, but not pain. Removal of the coral is painless, but why would you want to?
Texturally, if fells like a dry, non slimy amoeba.