This is book number 111 in the The Boxcar Children Mysteries series.
The Aldens are on vacation in Seattle! On their first day of sightseeing a mystery falls right into their laps when Benny finds a riddle taped to the bottom of a table. What does the riddle mean, and where will it take them? Follow Henry, Jessie, Violet, and Benny as they crack the mysterious riddle on their way through the Emerald City.