Congrats, really - that's
an amazing project, one that will go down history so you've really got the chance to "say something". Is that what you want to do or would you rather just entertain? Here's a way of doing both:
I haven't checked your predecessors out so I don't know what's been done already but one thought that comes to mind is to make it an experiment in telepathy. You announce (beforehand - call a journalist:) that while you're up there you will be thinking intensely of a particular word/image/song (or whatever, but make sure you state clearly beforehand which it is) - perhaps even one that you have beforehand deposited safely where no one can see it - and that you are hoping to be able to transmit it telepathically to say thousands of people. Afterwards you will of course be asked what it was that you were "transmitting".
So what was it? Well that depends on what's important to you. If it was me up there I'd choose to make it about animals' rights (but I'd choose a lovely image, not a horrid one) but that's just me and that's just an example to explain what I mean. What I'm saying is that when you're asked to reveal "the correct answer" you have the chance to influence people, because many will be listening to see if they were correct i.e. if the image they received was the same one you transmitted.
BTW I guarantee that thousands of people
will be right...