"Send your bread out upon the waters," says the Teacher, and eventually it'll be returned to you. (This is part of an argument for generosity, and "upon the waters" probably doesn't actually mean that you need to send your bread over the sea. If you spread the love, you'll get it back. Simple.)
No one knows the way God works, just like no one knows how he puts the spirit into the body.
Ecclesiastes says, again, that whatever comes to mankind is still—brace yourself—vanity.
Don't Worry, Be Happy
Ecclesiastes tells young people that they should live it up, and enjoy their youth while they still can. Days of darkness are soon to come. (This guy must've been fun at parties.)
Even though God's going to bring you to judgment (again, Ecclesiastes doesn't seem to have any idea how this happens, since he doesn't believe in an afterlife), you shouldn't worry about any of the things you do.
But even though youth is the best time to be alive, it's really, when you think about it, just more… vanity.