1. My collection of paper journals & diaries. Or better yet, get at least a dozen or more people to contribute journal entries that can be combined into a single book which shares not just my life but others from this time frame as well. Why? Because it's something I would want to find. I love reading journals, published or not, as well as historical letters. They show how things can change but that the concerns & fixations of the mind - the heart of things that matter - remain constant.
2. If I'm the one who gets to open the box, I would like there to be a letter to myself about what my life is like at this very moment, how I feel about it, where I am & what I'm doing as well as I what I hope for my life to be like fifty years from now (at the age of 80!) - ask myself in that letter if I managed to do anything I wanted to do, and if I hadn't, to do as many of those things a.s.a.p., no excuses. Why? I should have done this years ago, honestly. & imagine being able to read it, look back and recall all that I likely would have forgotten? All the things I thought so important, so dreadfully hard, put behind me & in perspective.
3. Photographs of various places, things & people. Why? In 30 years I would hope that the photograph will still have the curious charm and intrigue that ones from 50 years or more ago do today. It would be interesting to see just how much has (visually) changed, what will still exist or have passed away.
Because the first two points are quite similiar, I'm adding a fourth one.
4. All of my top favourite songs on an ipod shuffle in the hopes that this, by then, ancient device will still be playable. Why? I love a variety of music - some I discovered on my own and much that was shared - and it would be something to listen to while looking through old photographs and pages of writings.