A wander for the traveler who reads the plaque and keeps walking: three places whose ordinary setting gives away nothing about the spectacle inside — a blank Valletta fortress that erupts into wall-to-wall baroque gold around a signed Caravaggio, a full-scale Parthenon with a gilded 42-foot Athena marooned in a Nashville park, and a small Nebraska museum running a fiber-optic river of Kool-Aid.