Children usually have some interesting or amusing tale to tell you that's happened to them in the past. The 'When I was younger...' or 'When I was really small...' banner often produces good ideas for stories or poems. A good starting point is 'When I was little I used to think that...' - children usually love demonstrating how much they've advanced! :-)

Give the children a list of some everyday items such as a shoe, a door, a spoon or a window. Their task is to write the story of how that item was invented. They need to think when the item was first invented, who invented it and write the story of the 'why and how' the item was invented. They should think about the reactions of others to the invention and they may wish to write about how the item became as commonplace as it is now.
A similar thing can be done with discoveries. An interesting point of discussion might be what is the difference between and invention and a discovery.
A good follow-on activity, of course, would be to research details of how the things were really invented or discovered. You could also explore myths from various cultures that deal with things like the discovery of fire etc.