I suppose I'm a bit literal minded so the first thing I would send are mummified corpses of a woman and man. (so they could study the physical aspects of humans)

The second thing would be audio and video recordings of humans speaking as much of and as many human languages as could be included onto some kind of video/audio player and storage medium. (which would also serve as way to show our technology creation abilities)

The third thing I would send would be frozen or preserved examples of the kinds of foods that humans eat.

My opinion is that language/technology, how our bodies have evolved, and the kind/styles of food that we eat are the most unique aspects of being humans.

Thanks for this interesting question. :)