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.

A waffle house

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

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.

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

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

@YolkBrushWork402 - A giant florescent dildo.

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

My 1952 soviet bulgaria coin

A single horseshoe crab, naturally deceased.

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

A dinosaur skeleton

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

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

Dildo

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 black monolith

A primitive water bucket, made of Moon minerals.

NASA: "We Found Evidence that Moon had Water, and Life!"

@YolkBrushWork402 A human skeleton.

Mars bar

purely to confuse NASA scientists

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

A "Kilroy was here" graffiti.

A German flag colored towel

That missing sub.

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

A throne with a skeleton decked out in ancient roman clothing.

A post with the Liberian flag attached to it.

SOLD OUT

A statue like the ones on Easter island!

1960's era microwave oven.

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.

A pen lid with the long bit broken off, a shopping trolly token, 1 threadbare sock, a novelty rubber (eraser), and one of those black twisty wires that are used to tie up new USB cables, all sealed in a sandwich bag with 'aubergine curry' written on it in janky handwriting.

Why all this random rubbish? Because it'll absolutely screw with their brains!

Penguin Originals: "An Epic of Gilgamesh".

If I would have to put it now to be discovered in - lets say- next week - black monolit from Space Odyssey

If i could put it back in time - replika of some ancient piramid with gliphs stating that human flow to Mars to help martians to build the pyramid

A Viking longboat.

A sign that says "Moved to Earth"

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

"They will look just like you. Do not trust them."

A boat. Specifically one made for taxing through canals.

bending the rules and allowing multiple objects i think a neckbeard nest would be super funny

A green 1987 Lada Niva.

Maybe a Roman coin hoard?

Rick Astley

Pioneer 10.

I'd get some clay, make some nonsense sculptures, then just dot them around. There'd be whole reports written on what a triangle with a dot on it could possibly mean.

A Martian globe.

Not one, but five General Motors TBM Avenger torpedo bombers.

a chemical toilet

I would put one of those rock faces 🗿. It would make nasa think ancient tribes had space travel capability.

Why not the Great Wall of china? Like not the exact Great Wall of China but a duplicate of it that is one to one scale. It would be clearly visible when people are looking at Mars with relatively strong telescopes so we would know it was there long before we were there

A few planks of wood.

Some Neanderthal skeletons mostly buried in the sand so that it looks like they were just uncovered by erosion

An old rotary telephone.

A hot pocket

a rocket perfectly designed for the martian environment, so that the rocket doesnt destroy itself on launch like the starship prototype for one example, and for humans to return home from mars.

I'm thinking either the golden disk from Voyager, or the puzzle box prop from Hellraiser.