Ooh, that's lovely! Go for anonymous, totally. I agree with Sara--I'd be so tempted to have some contact info, but it's definitely better anonymous. (It won't feel freaky in the "who is this guy" sort of way if there's no contact info, something there would be danger of if you had an e-mail addy or something included.)

If you do end up doing this, I'd love to hear how it went. If some random stranger did that to me, it would *totally* make my day, and I can't imagine too many girls who *wouldn't* find that uplifting, especially if, as Anna says, they're the sort who would make you think that about them.