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.
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!"
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 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
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.
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 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!
A 1950s branded household item, like a toy or something that is no longer in production and had a hard end date. I would place or within the path of the rover but in a place it hasn't been yet
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'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.