The inappropriate word is 'voyage'.
'River trip' is fine, and 'river cruise' is fine. The term 'river cruise' is often used for commercial holiday packages: for example, a luxury two-week trip along the Rhine or Danube, including visits to places of interest along the way, can be called a 'river cruise'. Open any magazine aimed at wealthy older people and you'll see dozens of river cruises advertised.
'Voyage' is not used for river trips. In fact, 'voyage' is rarely used at all in modern contexts. It generally only comes up in multiple choice questions to test whether students know NOT to use it. Because of its resemblance to Latin cognates such as 'voyage' in French, 'viaje' in Spanish, 'viaggio' in Italian or 'viagem' in Portuguese, students tend to assume that the English word 'voyage' has a similar meaning. Be warned - it doesn't! The only context in which we use 'voyage' is for lengthy and usually historical expeditions, often into the unknown: for example, to describe the voyages of discovery of earlier centuries (Marco Polo, Christopher Columbus, Charles Darwin) and when referring to space travel in a romantic, rather than scientific, way.
You can go sightseeing when you're on a trip or a cruise. 'Voyage' is not an appropriate way to refer to modern-day tourism involving sightseeing. If your students were to use 'voyage' in this context, it would be seen as incorrect.
I hope that helps.