From Mark of the Pixie
Sometimes I will stat up a terrain (i.e. the swamp of nightmare) as a monster to make travel interesting.
Over the hours, days or weeks it will harass the PCs with its attacks (swarms of insects, thorns) and use its special abilities against them (bog pits, gas fires, nightmares, paranoia).
It is mindless, and the rounds are normally hours or days, but it puts the conflict for survival into a familiar form.
PCs might need to defend with wilderness skills, and there is no way to attack or injure the swamp.