People have been doing this for a while now; in their tombstones. I live next to a cemetery, sometimes I go there and read some. Most are written by grieving families - a couple phrases and that's it. The ones written by their occupants are invariably the most interesting ones.

You could even make it a bit of a puzzle: "There is a secret message in your great grandparent's tomb, but only visible during the Summer Solstice, on sunset". It would require some math and some careful placement, and durable materials. It could be a nice activity for your offspring.