The Secret Box, by Barbara Lehman (2011), is a very rare type of book indeed-- a wordless picture book about magical journeys through time and space.
"What if a child's treasure box from the past could provide...a connection between people, places, and time?" wondered Lehman. And from that idea came a wordless picture book that is either hauntingly mysterious and wonderful, or confusing. Or both, which is fine--if there's enough of a hook (which I'd argue there is here) confusion leads to slower re-reading, savouring the details, and engaging with the illustrations to make the story one's own.
A long time ago, a boy hides a box beneath the floorboards in the attic of his school. Time passes, and around the school a city grows...and one day, up in the same attic room where their beds are, three boys find that same box. They open it, and inside they find clues--pictures, maps, and an old postcard--that lead them to a magical place outside of normal time, Seahorse Pier.
There at Seahorse Pier they are welcomed by a crowd of boys from ages past. And then the book ends with two more boys, finding the same box, and reading on the postcard, "Greetings from Seahorse Pier." It's easy to bet that they will soon be off on the journey themselves....
I think this a lovely springboard for the imagination. We never know who these boys are...and so their stories become ones for readers to muse about on their own. Although I see no reason why a five or six year old wouldn't enjoy it, I think it is rather perfect for the older reader of picture books, kids like my own ten year old who still enjoy (and why not?) the quick flights of fancy they provide. In this case, the crossover to older kids is rather effective because it's very much like a short graphic novel, requiring much the same concentration from the reader to meld the visual with the story. At any event, he's read this about five time since we've had it.
You can find more pictures from the book at Seven Impossible Things Before Breakfast, and here's another review at Waking Brain Cells.
added bonus: I get to put a "reading in color" label on this one, thanks to the kid shown in the picture above, who's one of the main characters. This bring me up to two fantasy picture books in my multicultural sci fi/fantasy list (above). (The other is the lovely Chalk, by Bill Thompson). I've never actively looked for these, but do leave me any recommendations you may have for others.