To my American ear, "a voyage of discovery" sounds like something the Jacques Cousteau Society would organize!
A field trip(una gita scolastica) is what American students go on.
In AE, I would simply say "A field trip to Florence."
I think it is understood that the students will be "discovering" Florence.
A "field trip" to my BE ears would imply a trip that a specific department of students would go on e.g. history or art students going on a trip to learn about their specific area of study. In BE we would say "A school trip to Florence" or, if you really want to be fanciful , "A school trip discovering Florence".