Croatian Vacation

Anything other than flying is going to be time-consuming. How much time do you have for the Marseille-Croatia part of your trip? How much time are you allowing for Croatia itself? What stops will the cruise have made?

I would fly there and spend more time in Croatia.
If you take trains you would need to get into Northern Italy, Milan, Venice. From Venice there are large ferries and bus shuttles.
Where will you visit on the Viking cruise?

We are on a Viking Cruise starting in Lyon and ending in Avignon. We have about 6 days to spend in Croatia. Trying to determine the best town to fly into.

Do you plan to rent a car once you are in Croatia? What are your main interests in seeing and doing there?

Fly and then rent a car. Recommend visiting Rovinj and the nearby hill towns.

Our trip was back in 2004, and we flew into Venice, Italy, and rented a car. We drove to Croatia, by way of Slovenia, and turned the car back in in Venice.

If you’re focusing just on Croatia, then perhaps flying from Marseille to Split, Croatia’s 2nd largest city, is the way to go. In Split, see Diocletian’s Palace, where the emperor of Ancient Rome retired in luxury. We stayed just outside Split, in Solin. From Split, you could venture just west, to Trogir, and north to the capital, Zagreb. Between Zagreb and Split is the unique and phenomenal Plitvice Lakes National Park. South of Split, along the coast, is Dubrovnik. There are also a lot of islands just off the west coast of the Croatian mainland. Depending on how much driving you want to do, maybe a trip to an island, and whether you’d look at staying several one or two night stands, you could include all these in an itinerary over about 6 days.

Volotea Air does cheap, direct flights between Marseille and Split on Wednesdays only; otherwise you could fly other days on other airlines, but with longer, connecting flights.

We spent two weeks in Croatia last fall, and felt like we barely scratched the surface. Sorting out what you're interests are will help you a lot in figuring out your itinerary. We really liked Split for the museums, sights and restaurants. Plitvice is really beautiful and unique. Even if you're not a nature-lover, it's worth the effort to get there. Make sure you book in advance, as there are no ticket sales at the entrances. You can comfortably do it in a full day. We stayed overnight at one of the park hotels, Hotel Jerezo. It was a hoot. Straight out of the Tito era property: big rooms, 60's decor, easy parking and free second day access to the park, which is helpful if you arrive after lunch from wherever you're coming.

Of the big cities in Croatia, I enjoyed Split the most (although Zagreb was a close second). Split has interesting museums (the ethnographic was my favorite), a large Mestrovic museum/gallery, Diocletian's Palace, and some great restaurants. I much preferred it to Dubrovnik, which was a zoo. Too many people for small spaces was the biggest complaint we had. The wall is interesting and there are pretty views. Add a couple of churches, a couple of war museums/exhibits (the best being on Mr Srd near the lookout), and that's pretty much it. Not worth the effort IMHO.

I loved Rovinj and some of the hill towns are really cute (although I thought Motovun was overrated, both when we visited 5 years ago and now--but many people here will disagree). Driving in Croatia is really easy, but we spent more time in the car than we had planned for. Croatia is bigger than it looks.

It really depends on your interests. Museums and art -- Split and Zagreb. Beaches? South of Split and/or islands. Nature, definitely Plitvice. Hill towns with Italian influence and quaintness factor is Istria. Depends on your time, too. With a short visit, I'd see less and enjoy it more. Round-tripping to/from Split makes the most sense to me.

I'm a huge Plitvice fan and recommend it to anyone without mobility issues (there's a good bit of walking required), but it can be horribly crowded during day-tripping hours. The way to see it is to spend one night in or near the park so you can see it late on your arrival day and/or at opening the next morning. Doing that will make a really huge difference in your enjoyment of the park.

You may have more flight options into Zagreb, but there are certainly flights into Split and Dubrovnik as well as Zadar. I use skyscanner.com to look for intra-European flights. You don't have much time, so it's essential that you fly into and out of airports as close as possible to the places you want to visit. I would not try to see both northern Croatia and southern Croatia in 6 days.

Investigate flight from Marseille to Pula if known - all airlines in chaos and next year uncertain.
Rent car in Pula, at airport. drive into the town to Arena (colosseum) for a visit and a coffee, then drive north
on a circular tour of Istria, including the hilltowns of Oportalj, Grascisce, Motovun, then Lim Fjord,
back to Rovinj for at least two nights (evenings are special there) then down again to Pula airport,
which is helpfully north of the city with a tiny yellow sign saying Zracna Luka. You could use Rovinj as your base and do all of Istria in day trips. Istria has a character all its own, and is not far by air from France. Connecting flight back to Marseille?
If not Istria, there will be many flights to Split and even Zadar, based on this year's rapid recovery of routes.

if you get to Rimini from Marseille you can make your way to Venice, Corfu and finally the Dalmatian Coast, you can do that with a sailboat.

Volotea has seasonal service to Split from Marseilles, Nice and Lyon - at least for 2019, but things might change for 2021.

i actually did the route that DeluvianLex claimed a few years ago. anything is possible but if you rent a car instead of a boat, you can enjoy your time in Rovinj or Pula, go to Venice into Milan and Genoa ( going to be quite a trip though!)