A lesser-known one I adore is 'The Adventure of the Musgrave Ritual.' It's Holmes solving a centuries-old family ritual as a treasure map. A butler vanishes after obsessively studying the ritual. Holmes treats the archaic verses as a set of surveying instructions. His genius is in applying literal, geometric logic to poetic lines, pacing out distances under the elm tree and the oak.
It's pure puzzle-solving, disconnected from a contemporary crime at the start. He finds the ancient crown of the kings of England. The story highlights his love for intellectual challenges for their own sake and his encyclopedic knowledge of obscure trivia. It's a beautiful demonstration that his mind is a repository of strange knowledge, ready to be applied in unexpected ways.