A "Voyager" type spacecraft with a Golden Record but with contents depicting a civilization of Cats in Mars, and the Space Craft + Golden Record are made from materials from Mars. Put this in orbit around Mars, and have it emit a signal on loop with an Arecibo Message depicting the Cat Civilization.

The scientific community would just collapse and we'd be worshipping cats as a world wide religion.

😸

-Sincerely, totally not a cat

A teapot in orbit around the sun somewhere between Earth and Mars.

A dinosaur skeleton in a spacesuit.

That manhole cover from Operation Plumbbob.

Russell's Teapot.

A radio station streaming "Baby Shark" on all frequencies

Gonna steal an idea from Karl Pilkington and leave a dishwasher somewhere on Mars for one of the rovers to find.

A giant ship that can transform into a giant robot maid, complete with a vacuum to suck planets' air.

"YOU ARE HERE" map.

Everything is already in space. Also, whale songs

Arts and Humanities research.

A lone white lawn chair on the surface of Mars

Moon sized amoeba.

I'd suggest 35,000 low-earth-orbit satellites but Elon is already working on that.

Ghosts

A single protein molecule.

Something subtle enough to spark debate and controversy but not overplaying it like putting an entire living thing from Earth would be.

Sprinkle some water on the moon

A very small probe that emits an impossible to miss signal at random pretty long intervals and then adjusts its orbit to be very different. It would basically pretend to be various Aliens civilizations trying to contact us. Like every 45 days a new signal from a new part of the sky is picked up and then never repeats and if were just getting tons of messages at once. And since the probe is small and doesnt have a predictable orbit it would be very hard to figure out that its actually a probe and then find it.

Alternatively. A plane full of snakes.

Dark matter, duh.

TMA-2 / Zagadka / Big Brother (the Jovian monolith).