Seems it would be possible to have it spin on its axis so that it reflects the light from the sun towards Earth in some kind of pulsing flashing manner. That would attract attention. No need for electronics. Of course encoding a message in the flash would be almost impossible, but it could serve as a beacon for a future civilisation to attempt to go up there and retrieve it.

Maybe facet the mirrors so that the reflections would flicker in some something like Morse code. I imagine it would be very difficult to decipher and could only convey a few bytes though.

Maybe just send a sealed capsule capable of re-entry into an orbit that would actually intersect earth in a few hundred thousand years. I doubt we're capable of that sort of precision though..

That's a brilliant idea. I think the electronics part is doable, a radio transmitter is a very simple thing. Some metal alloy with sufficient robustness would probably do the job.

We could even build an analog device that could store the messages just like a music box cylinder.

https://www.google.com/search?q=simple+radio+transmitter&tbm...

https://www.google.com/search?q=music+box+cylinder&tbm=isch

Also check out Graveyard orbits for decommissioned satellites where they can stay for millions of years and be our monuments from the 21st century.

https://en.wikipedia.org/wiki/Graveyard_orbit

Space is a pretty hostile environment over those timescales.

Selecting a power source is hard as solar cells almost certainly will not last more than a hundred or two years. Nuclear materials with long half-lives don’t provide much power.

Electronics would just have to be big to avoid radiation damage from being a concern.