The phrase is "bonan vojaĝon", and not "bona vojaĝo". The accusative indicates the presence of an invisible verb. The full sentence is something like "[I hope that you have a] good trip", with the bracketed part typically left unsaid. We do this in English when we say things like "good morning", which really means "[I hope that you have a] good morning". But, while we customarily say good morning in English, we do not wish someone a good trip often enough for the verb to be understood without being said.
As not a native English speaker I would like to know the difference between voyage, travel, tour, trip and journey. When to use them? Is voyage the only one accepted here? Are there esperanto words for each of them and for same usage? (btw Godspeed is a new english phrase for me, thanks!).
As usual in English, there aren't hard and fast rules. Almost all of your suggestions would be understood in context, interchangeably.
Broadly, you could refine each to:
Voyage - often used for sea travel
Travel - any kind of movement. "How do you travel to work each day? Car, bike, bus, or train?"
Tour - a circuit of an area, seeing lots of different places, and returning a different way fron the path you left by. Tour de France.
Trip - generally of a short duration, from which you return. "I just took a weekend trip up to the north coast."
Journey - generally of long duration, possibly without returning. "The journey to their home in a new land would take many months."