I would draw the iconic S on the ground.

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The flag of North Korea.

Something simple, like a bolt from one of the rovers. But make sure it's from one that has never been in that half of the planet.

An orb with a print for a three fingered hand.

A copy of ET for the Atari 2600

Elon Musk

A half-eaten tuna sandwich. By the time they manage to retrieve it for examination, it would definitely be all soggy.

My body in a lawn chair, with my right thumb extended out for hitchhiking, my left hand holding an American flag, and a sign in my lap saying "California or bust!"

A waffle house

Not a single act, but a series of actions. The rovers power down during Martian nights, right? So, during these periods, you could rearrange the rocks nearby. At first, NASA might not even pick up on the changes, but eventually, someone's bound to notice.

Imagine the buzz it would create! The scientists studying the photographs sent back by the rovers would be left scratching their heads. They'd probably first suspect some unknown natural phenomena, or even the rovers themselves, until the changes became too orchestrated to explain away. It would be a cosmic puzzle, a Martian mystery that would have them combing through data for answers.

No extraordinary or unnatural objects, just ordinary Martian rocks moved around. Instead of presenting them with an object that screams 'alien intervention,' you'd be nudging them into a mystery, all with the Martian surface's naturally occurring elements.

@YolkBrushWork402 - A giant florescent dildo.

A giant head of Lenin. It’s a red planet after all.

A "Welcome to Venus" sign. Just imagine their faces, they'd be so confused...

My 1952 soviet bulgaria coin

A single horseshoe crab, naturally deceased.

A stainless steel commemorative plaque that says "Site of humanity's first successful time travel experiment - March 13th 2153"

An altered skeleton, half buried. Give it a larger than normal skull or extra long arms or toes or something like that.

I'd put an exact replica of the NASA HQ on Mars.

Assuming size is not an object, a pyramid; doesn't matter if it's Myan, Aztec, or Egyptian, heck make it a mix, and have it buried in a mound of dirt or in a mountain with just parts of it sticking out.

Or if we aren't going supernatural teleportation of items, I'd think something like the person who said moving rocks around, maybe lay rocks out in shapes like roads or the foundations of houses.

A dinosaur skeleton

A sealed hab dome, containing a fish breeding tank that outputs a portion every so often into another tank, and a flock of penguins. There are also penguin shaped space suits mounted next to the air locks

A "Kilroy was here" graffiti.

A simple message: "ALL THESE WORLDS ARE YOURS EXCEPT MARS, ATTEMPT NO LANDING THERE."

That missing sub.

I'd put an empty spacesuit from the Apollo era.

@YolkBrushWork402 A human skeleton.

Mars bar

Dildo

A post with the Liberian flag attached to it.

A German flag colored towel

purely to confuse NASA scientists

Ah then it is simple: we place an ESA spacecraft there.

Something that's flying around here as well, and often. Maybe they wouldn't even realize that it doesn't belong there at first. Like a condom wrapper or a dog turd.

The Ozymandius Statue, with the exact quote from the poem.

A private property sign.