Not long ago I took out of a cache a vial full of cologne - one of those little bottles they give as samples.
The cache was in a tree in the largest green area of the city (think Central Park). The cologne fit with the romantic theme of the cache, but wild animals (mainly birds, snakes, rabbits, squirrels and other small rodents) do live there, and food or scented items can disturb them. So it wasn't really trash, but it didn't belong in a cache either.