excursion vs mystery tour

excursion

noun
  • A brief recreational trip; a journey out of the usual way. 

  • An occurrence where an aircraft runs off the end or side of a runway or taxiway, usually during takeoff, landing, or taxi. 

  • A deviation in pitch, for example in the syllables of enthusiastic speech. 

  • A wandering from the main subject: a digression. 

verb
  • To go on a recreational trip or excursion. 

mystery tour

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

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