I really like landscapes. but I hate them. I see things like this and this and it just takes me back. I just get reminded of the fact that there's so many beautiful places in the world, and I just think that it's just amazing. But, then I realize that I'll probably never see a lot of these places, so then I just get really disappointing.
I love landscapes, I would definitely show the alien _some_ piece of art that focuses on the natural world. Perhaps earth as seen from the ISS or something, showing we're capable of space-flight, aware of the fragility of our existence etc. Also: How old are you? You don't need to answer, I just want you to think about your life not in terms of "In 20 years I've not explored the world" but rather, "I have 20 years preparing me to go exploring when I'm ready". I'd argue that at 20 you're likely to have another 60-80 years ahead of you in order to do so... but that may not be true. Scientists are literally working to cure aging, and who knows what discoveries we'll make in the next half a decade.