I think the obvious answer in the modern world would not be a phsyical one, but some sort of measure of wealth inequality. At some point, if too few have too much it destroys a country from the inside out over the long run. It does far more damage than any tsunami ever could. I don't have a number or exact measure in mind, but that would be the warning I would leave to future generations.

Depends on how metaphorical and/or political you want to get.

Arguably books could be considered warning waystones, but that's a stretch in this context.

Physical monuments though, we have loads, lots of war memorials are/were intended as warning about the cost of war.

Auschwitz-Birkenau being left as as it is could be considered another.

If you want to get really close to similar intentions there are the long term nuclear waste warnings:

https://en.wikipedia.org/wiki/Long-term_nuclear_waste_warnin...

A bit more esoteric (and less warningy) and you get the signals we send in to space intentionally as a time-capsule/marker for potential alien contact.

That suggests a possibly better strategy (though very long term): pepper the portion of the landscape believed to be safe with "it is safe to build here" monoliths, each as stable as a typical building, and over time only the ones that speak truly will remain.

Would work for volcanoes and earthquakes as well.

Do you have a citation for this? The most Gemini could say is: "While research has not identified a specific tsunami stone located at the Fukushima Daiichi site that was directly violated, the spirit of these ancient warnings was undeniably ignored." (https://aistudio.google.com/app/prompts?state=%7B%22ids%22:%...)