What could we build that would last millenniums should we ever face extinction or near-extinction

Something that could withstand nature and destruction by survivors and their children's children who would undoubtedly be cast back into the stone age and who would have very little concept of the knowledge we have accrued. People that would, at times, seek to destroy, cannibalize, pilfer or even simply try to posses it for themselves.
Something that all can see and would be very hard to hide. Something that would be written in the universal language of mathematics and point toward astronomy and the earth's geometry.

Giant pyramids seem to work pretty well.
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If we make a giant bucky ball and bury it on the Moon, then whomever finds it will know a technologically advanced civilization was once here.

Anything on Earth would erode away and be easily accessible to people with primitive technologies, any orbit would decay and be subjected to micro-meteorites, cosmic rays, solar wind, and orbital decay. Anything exposed on the Moon would be subject to micro-meteorites, cosmic rays and solar wind so burying stuff on the Moon would be best. Something like a bucky ball or graphene based material demonstrates nano-technology and makes for a very tough structure, and if whatever is contained is kept in a noble gas environment, there should be no degradation.

Then there's the old trick of trying to encode a message in DNA as it's really the only thing that's self repairing.

So you want something that can house some 7 billion people, animals, plants, housing, water, food, parks, recreational areas and all those things that we humans like to have around us.

Maybe a spare planet would be nice.

Some of the Pioneer & Voyager probes will still be travelling in Space, undamaged.

A large "Noah's Ark" space station in High-Earth-Orbit?