That is great of you to do that for the future owners. If you leave the link to the forum, that will be the biggest help of all.

I'd leave an instruction sheet for how to backwash your filter. Also leave the book for the pump/filter and heater and also a note of how old each of them is. Maybe a brief sheet with pool opening and closing directions. Leave a set of test results taken by you at the last possible moment so they will have something to work with.