Introduction
You are on an expedition to found a colony on Mars, when from a nearby crater, a group of Martians suddenly emerges. They seem eager to communicate, but they’re the impatient kind and demand you represent the human race. What do you share with them to show that humanity is worth their time?
Nothing brings people together like good food.
I've found that food, can often show something that nothing else would show. If I, an 11-year-old, is able to get on Mars, I'm sure some other normal food from earth can. If not, I would invite them to earth, to treat them. Cooking and baking is simple. Using the right amount of ingredients, and cooking it for the right amount of time. It could be hard to master the recipe, but after sometime, you could practically memorise it. It also takes a while to make, at least most things. Requiring patience. I mean, I wouldn't spend 4 hours to make a cookie for my worst enemy.
The book and it's cover.
That being said, there are still different types of aliens. There could be kind ones, or smart ones, each portrayed by body language and maybe the way they're dressed. You should never judge a book by it's cover, but in this case, the book is sealed in those plastic bags, so you can only judge it by it's cover. In my opinion, cooking shows a lot. It can show how much you care about someone or something, and it can show things about you too. How patient you are, and how much you are dedicated to do things (I only buy 5 blocks of butter every so often to make cupcakes).
Kindness, and patience.
So, in this scenario, what you want to show is that you match with them. To show that you are just like them, and possibly better.
For a kind alien, you could teach them. If they like it, of course. I, would show that we could be worth their time, because we all have the patience to teach, and give to the others. It could grasp their attention, and we could also gain some of their knowledge too.
Science anytime, any day, and every day.
There would probably be smart aliens too, being able to survive on such a planet, with cold and harsh conditions, and lots of dust, (We don't really know what they need in order to survive, but our common knowledge is that anything that lives, needs nutrients and warmth.) Cooking, is basically chemistry. The way different compounds react to different energy sources, whether it is heat or cold. The people who cook are and create recipes, typically need an understanding as to how and why it works that way. We're pretty much inventing new compounds, using science, and dealing with energy sources, in our daily life. Similar to what you might do for chemistry. Both end with a rewarding outcome, but cooking can lead to other effects as well.
For me, baking is the answer
This also shows personality. Not everyone likes cooking or baking, and that's fine. But it also shows me. I shouldn't be selfish and have the focus all on me, but the truth is. If I encountered them, I would want to show “me” as well. I'm a person who loves cooking and baking, and I have a fond passion for it. I could share it with them, and it would be something that I, part of humanity, taught them. Showing that we are intelligent, kind, and willing to help others. Now, I hope they're hungry.