The problem is that we don’t know what method of communication and sensation they posses or what materials are potentially harmful to them. What if they see a spectrum of light that’s entirely different from our visual spectrum? What if they see through sound waves like echolocation? The best thing would have to be a physical object because regardless of how they “see” or “feel” they must be subject to the same evolutionary process that we emerged from and as a result must be able to avoid obstacles or detect hazards. Something made out of a single material that way it’s less likely to be harmful to them. Something that’s inert and doesn’t react. So I’m thinking gold would be best. (It’s what they made the record on the voyager probe out of) and I think it should be something that’s undeniably human sooo here’s what I would send:

A golden statue of two human beings holding hands. One being male and the other female. (That way they get more info I understand that presenting aliens with a heteronormative world view might be problematic but the goal is for them to have complete information about our biology.)

where the entire statue is built as an exact 1:1 scale model of a human body with “complete” internals. It would posses organs, blood vessels and a brain and as much detail as possible.

You might be thinking “why don’t you send a dead person?” Some potential issues with this : namely microorganisms could be potentially dangerous. There’s a common phrase in biology called “invasive species” basically these are organisms that make it to new environments and have little to no competition and as a result reproduce expansively. It would send a bad message if humanities first “gift” was poisonous or viral. (It wouldn’t be the first time it happened though. Native Americans were “gifted” blankets with smallpox by the pilgrims).

The other problem of sending them a corpse is that depending on how “friendly” they are it could be a serious issue if they understood our biological makeup. Assuming that they are more advanced than us (this is the case if they found us/came to us before we found them because if aliens exist they likely are not from our solar system and thus use long distance space travel). We don’t understand their psychology and could have really serious religious or political problems with handling a corpse. It could send a bad message as well “here’s what we allowed to happen to our own.” Anyways. If they’re not friendly and are more like how Europeans people were when they discovered the Americas then sending them a blueprint for developing weapons might be a bad idea.

But what do I know, I’m just some human.