Adventure Dice is a super rules-lite storytelling game designed, primarily, for kids to tell stories together – or for adults to tell stories with kids. (Or, honestly, just adults all together. It’s a ludicrously simple system with a lot of output.)
It comes with four dice that you can cut out and stick together (or that you could reproduce on other dice). It’s pretty freeform, and it’s more interested in collaborative narrative than hit points and armour classes. You can also use it as a means of generating stories for writing.