The trick is to punch as fast as you can and make contact with the bottle - without it making any sound or moving much at all. It's perfectly possible, but ridiculously hard.
You could always make a punching bag out of an old duffel bag you might have. Other than that i don't really know of anything that you can hit that wont damage the gloves or yourself. Could try tying a mattress around a tree, or any other sort of padding.
People.
You could shadowbox
i got a pair of boxing gloves and we are gettin some real nice sunny days, so does anyone know a way you can use your boxing gloves outside? what would you hit though if not a punching bag? any replacements outside?