I think the "one puzzle" is the best idea; it keeps the focus on design / gameplay, and you can interpret it to be a labyrinth or a room where you wait 20 minutes or whatever you want. But then it's also kind of general, which defeats the purpose of a contest to place constraints and foster creative solutions.
Sentura: Read up on the history of the labyrinth, first. Merk is obviously referring to the classical labyrinths - most commonly Greek / Roman / medieval.