I love this question. I cant wait to read some of the answers. The answer about the bible is very interesting and would be neat to put into a time capsule.
Well for starters I think I would put my writings and poems in the capsule, as they tell my feelings and emotions at the time. To compare them in years or even decades to come would be astonishing. To see the change in the world.
Maybe even technology, because either the world is going to crash as we know it, or the technology is going to be so far gone, that the world will get worse before it gets better.
So maybe a cell phone of today. I am sure that in decades a cell phone of today will be so old fashioned for the children growing up in a new era, they will have the same look on their face that some of us do when our children try to teach us about computers. lol. So with that said, yes I would let my grandchildren open it. It would be a great discovery for them.