Here's how it works. Make a fairy boat from twigs, grass and mud. Or whatever. Then let the kids play with it. It doesn't even matter if it sinks. Just laugh. Then, after lunch or during a quiet time, tell a story where a mouse (or a fairy, an ant, etc.) finds a small boat along the river and sets sail on an adventure. Don't have a fairy boat? Make a little house, or a fort. You can refer to a child's backpack, or a tree with a unique shape. Anything. Take 1-3 concrete things from your day and use them as anchors in your story. It could even be a piece of gum - imagine what an industrious little ant could do with a piece of gum!
This ties reality into the imagination, and vice versa, because after the story, the kids will inevitably want to play out some of the story line in their play, in which case they may need to make boat, in which case there might be a boat in the next story... This is the storytelling loop.