So I feel that it would not be useful to show them:
1. A piece of technology that we have; for if they have gotten here, they will have likely superceded it, and will in fact view us as technologically inferior.
2. A meal, a drink, a precious metal, something along that line; for if they see it, they can simply reason that they can take all that over our dead bodies, and in fact, it would be easier to do so.
Consequently, the only interesting thing we would have to a race of technologically superior aliens would be something they can never obtain, something about homo sapiens. So I would give them someone that can reason and think with the best of them, someone who can see a situation, think of it, and figure out applications or possibilities. While they likely possess such reason and intelligence, it would be a surprise for them to see something close to an equivalence in this. It is our best chance at sticking around, really. Or assimilation.