A statue like the ones on Easter island!

1960's era microwave oven.

purely to confuse NASA scientists

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

Pioneer 10.

A sign that says "Moved to Earth"

A Viking longboat.

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

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

Maybe a Roman coin hoard?

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

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

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 green 1987 Lada Niva.

Rick Astley

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

A boat. Specifically one made for taxing through canals.

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

SOLD OUT

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

Penguin Originals: "An Epic of Gilgamesh".

A primitive water bucket, made of Moon minerals.

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

a chemical toilet

A Martian globe.

A few planks of wood.

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

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

An old rotary telephone.

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 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.