Don't go the henna route. I work in a salon and I've never seen an example of henna'd hair that comes out anything like the person intended it to. And the problem with certain kinds/colors of henna is that you can't dye over it, bleaching it will make your hair break, or the henna will bleed through whatever you try to dye it.
The absolute best natural way to lighten your hair is squeeze lemon juice into your hair, slap on some sunscreen, and go hang out in the sun. It brightens your natural highlights, it doesn't involve any sort of chemicals or anything. I've seen this come out so beautiful so many times.
Good luck and let us know how it comes out!