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.

Oldest known stones date back to the early 1400s but there must have been older stones. Already those stones are mostly unreadable because of erosion and they are dated by secondary sources.

You can put warnings everytime there is a tsunami, which is "often" in Japan, but the issue is that a massive one like the 2011 earthquake and tsunami is a once in a millenium event so would indeed need to rely on very old warnings:

"The 2011 Tōhoku earthquake occurred in exactly the same area as the 869 earthquake, fulfilling the earlier prediction and causing major flooding in the Sendai area. [1]

Modern society is not good at this sort of very long term consideration and planning.

[1] https://en.wikipedia.org/wiki/869_J%C5%8Dgan_earthquake

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:%...)