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.

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.

With radiation half lives what they are, our society should be brainstorming how to segregate and mark nuclear waste storage areas. All areas that store radioactive waste.

Without clear warnings and boundaries humanity is just waiting for a catastrophe.

A tiny sign and words don’t count.