The Open When letters work as a variation of the Letters to the Future (=letters to be opened on an specific day in the future). There are letters written and given with instructions for opening at a later date or time.
There are many different types of Open When letter topics. They can be general ("when you miss me") or specific ("for your birthday"). On this post of Sharis' Berries blog you will find not less than... 280 topics! Plus some fun ideas and printable templates (like those on the picture, taken from the same post).