in one of my stories, two childhood friends bury a treasure box together when they are 10-12 years old and forget about it. What is inside the box may reveal an important development for the story. I haven't decided anything because I can't see what is in the box. I want the object turning the story to be an object children might treasure for real, and not something random just because convenient for the story. Did you or someone you know ever hide objects into a box? Did you fantasize about hiding things into a box? What were the objects?
Time capsules were a thing when I was a kid. 80s children would bury things in from the day, or things that were important to them like letters, toys etc. with a view to either digging them up themselves when they grew up or letting others dig them up.
You may be doing this in the wrong order. You probably want to know what the story is to figure out what that pivotal piece is. If you're a discovery writer, that may mean leaving a placeholder for now and filling it in later. Otherwise, if you really need to fill the box first, the answer would depend on a number of factors. Who are your characters? What era is it when they box up their time capsule? Where do they live? Are they from wealthy families? If we know these things, I bet we can come up with some good options.
I write a bit differently that other people and find myself thinking about "order," to use your word, quite a bit. I begin with philosophical themes in mind, and write stories around that. So my stories are really more like fables or parables. If I were faced with the challenge O.P. has given him or herself, I'd ask myself "what lesson do I want the characters to learn by the end of the story?" Then I would work backwards from that: what could I put in that box when they're ten that will later come back and teach them something.
I doubt that 10-12-year-olds would bury items which they found particularly valuable at that time, as they'd want to keep those items. They may choose a toy which used to be meaningful but they'd since grown out of. How about including a few pages of text, a short story they created together. The story could include a clue, a forgotten fantasy.
I used to do this at my grandparent's house, which we visited every summer. I'd write a note to my future self and tuck it in an old glass bottle or something I found out in the woods. I might add a few small toys, or cool looking pebbles or something I had found that summer. Then I'd bury it and remember where I put it and dig it up a few summers later.
