Since somebody already laid out the groundwork for situations like these, I guess I would start with that.

Once they know our numbers, we can hopefully start defining operations like addition, multiplication, and exponentiation, then move on to algebra and calculus. Great, so now they know we know some math. To show them we know science, regurgitating scientific knowledge might work, but I think they would be slightly more impressed with a prediction that utilizes that knowledge, followed by observation and confirmation. The number of seconds it takes an object to fall from a given height, calculated beforehand and then measured, or the number of days until some celestial phenomenon, represented by, for example, making a pile of pebbles for each unit of time you expect the event to take and taking away one pebble at each interval (expecting the punchline to occur at no pebbles), would be worth considering as attempts.