Zagreb to Florence

Lets see, you don't want to fly and you don't want to drive. That leaves....train?
10 1/2 to 11 1/2 hours, maybe a night train.

Then the option is, as mentioned, the train. The best route would in my opinion be via Ljubljana-Trieste-Venice.

There is a 12 hour Flix Bus overnight option, transfer at Bologna in the small hours of the morning. Hardly the most enticing looking journey, and that's being kind, but might as well put it there as an option, if only to be complete.

The most sensible train on this route seems to be a night train-
1810 Euro Night to Zurich, change at 0920 to the 0933 for Milan, change there for Florence arrive 1504;
The other night train is Zagreb depart 1250, Villach arrive 1708, depart 0055 to Bologna on the Night Jet arrive Bologna 0620, change for Florence arrive 0739.

The day train is at 0705,change at Jesenice and arrive at Nova Gorica at 1304, self transfer to Gorizia Centrale (Italy) for a train to Venice Mestre, change there for Florence arriving at 1939.

Both Die bahn and Rail Europe agree on these less than ideal routings being the only ones available.

It would cost more, but I wonder whether GoOpti offers shared-van (or non-shared-van, for more money) service from Zagreb to a convenient Italian city like Trieste or Venice. I know they run from Ljubljana to Venice. Once in a major Italian city, you'd have lots of rail departures to Florence. Via the fastest Freccia trains you'd have a 4 hr. 23 min. trip from Trieste or a 2 hr. 13 min. trip from Venice--or about 2 hr. if you could get GoOpti to drop you off at the Venice Mestre station.

If GoOpti can only help you from Ljubljana, there are both trains and buses from Zagreb to Ljublana. I think the travel time is about 2-1/2 hours. However, by the time you piece together the train-shuttle van-train routing, you might not have saved much time.

I'm always looking for cheap options, but this might be a situation where it's worth it to spend a little more money on a shuttle to get you into Italy and avoid that long slog of a train trip. Zagreb's 142 miles from Trieste; I wonder how much a taxi would charge to make that run, compared to GoOpti (if available)? Since Croatia's admission to the Schengen Zone, there shouldn't be a risk of getting hung up at the border.

Since Croatia's admission to the EU,

Technically, since their admission to the Schengen Treaty Zone. Switzerland is not an EU member, but they are a member of Schengen. Ireland is a member of the EU, but not a Schengen member.

You're right, Sam. I stated that incorrectly (now corrected); I do know the difference, because it causes me massive problems every summer, due to the 90-day limit.

Thanks so much for all your helpful suggestions! We are looking into each one of them!

If GoOpti can only help you from Ljubljana, there are both trains and
buses from Zagreb to Ljublana. I think the travel time is about 2-1/2
hours. However, by the time you piece together the train-shuttle
van-train routing, you might not have saved much time.

Maybe not much time, but a bit of money. A train ticket between Zagreb and Ljubljana costs €9 if bought in advance, and it is partly a rather scenic ride.

The big question is how much time you have between Zagreb and Florence? If you just want to get to Florence as fast as possible, I'd suggest the night train route mentioned.