Well above the nuclear waste, bury a tablet that teaches a language in a few different ways, like the Rosetta stone but designed for teaching from scratch. Add some easy texts for study. Add more complex texts to study after the easy texts. Add an elementary, qualitative intro into the ideas of nuclear physics. Now explain the danger of the buried substances in a sensible way!
(Better yet, build a breeder reactor, burn the "waste" as the nuclear fuel it is. Stop being deathly afraid of reprocessing plutonium, at least in the nations that already handle it and have nuclear weapons for last 60-70 years anyway.)