Rainstorm by Barbara Lehman
"It can be lonely sometimes on a rainy day in a big house with no one else around and only the quiet to keep you company. But if you find a key, a mysterious key, that leads you to an unexpected place . . . chances are your afternoon is about to get a lot more interesting." (front flap of book)
Rainstorm is a wordless picturebook that is a lot of fun. The pictures are able to tell a complete story of a little boy who is bored on a rainy day. He lives inside of an enormous house but is able to escape into his imagination and away to a lighthouse all with the turn of a magic key in a box with a ladder.