die Rundreise
round trip
A trip on which several places or sights are visited and which ends back at the starting point.
Wir planen eine dreiwöchige Rundreise durch Italien.
We are planning a three-week round trip through Italy.

round trip
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round trip
A trip on which several places or sights are visited and which ends back at the starting point.
Wir planen eine dreiwöchige Rundreise durch Italien.
We are planning a three-week round trip through Italy.

The word is made up of 'rund' (round) and 'Reise' (trip/journey). So it literally describes a trip that forms a circle and ends back at the starting point.
Unsere Rundreise durch Skandinavien beginnt und endet in Kopenhagen.
Our round trip through Scandinavia begins and ends in Copenhagen.
'Rundreise' (round trip) is very often combined with the verb 'machen' (to make/do). You say 'eine Rundreise machen' (to take/go on a round trip).
Nächstes Jahr möchten wir eine Rundreise durch die USA machen.
Next year we would like to take a round trip through the USA.
To describe the region being traveled through, the preposition 'durch' (through) is usually used. If you are currently on the trip, you say 'auf einer Rundreise sein' (to be on a round trip).
Sie machten eine unvergessliche Rundreise durch Vietnam.
They took an unforgettable round trip through Vietnam.
tour
'die Tour' and 'die Rundreise' can both describe a trip with several stops. 'die Tour' is broader and can also be short, for example a bike tour or a hike. 'die Rundreise' sounds more like a longer planned trip, often through a country or region, where you return to the starting point at the end. For a small excursion, people usually say 'Tour'. For a bigger trip with several places, 'Rundreise' is more precise.

Im Sommer planen meine Eltern eine Rundreise durch Italien.
In the summer, my parents are planning a round trip through Italy.
Nach dem Frühstück starten wir eine kleine Tour mit dem Fahrrad.
After breakfast, we're starting a short bike tour.