mystery tour vs walkabout

mystery tour

noun
  • A short-duration outing by coach to a destination that is not revealed in advance 

walkabout

noun
  • A walking trip. 

  • An absence, usually from a regular place with a possibility of a return. 

  • A nomadic excursion into the bush, especially one taken by young teenage boys in certain ancient-custom honoring tribes. 

  • A public stroll by some celebrity to meet a group of people informally. 

How often have the words mystery tour and walkabout occurred in a corpus of books? (source: Google Ngram Viewer )