Slovenia/Austria/Switzerland Trip May '24

You can catch the train to Salzburg from Innsbruck in less than 2 hrs.

Then you can catch the train back to Innsbruck and on to Zurich--5 hrs. 45 minutes total. It's one beautiful ride too.

So you have nine nights on the ground, correct? Have you been to Europe before?
In Switzerland, I'd go to Lucerne, not Zurich , as it's a much more charming city. However, the draw of Switzerland is the fabulous Alps, particularly the Berner Oberland.
Three locations is the most I'd recommend for nine nights.
Block out your nights and research travel times between locations, so you aren't spending too much of each day on the train.
Also, based on last year's pent-up demand, I'd book lodging asap. You're a little late for May, so a priority is to securing places to sleep .
Have a great trip and congrats on the wedding! Safe travels!

Yes 9 days on the ground. Have been to Europe before, but never to this area. Switzerland was actually just an add on based on the best flight options, I plan to re-visit Switzerland on a separate trip. Goal for the trip was really Austria & Slovenia.

Good to note we could day trip to Salzburg given its a quick train ride from Innsbruck.

Just going off the top of my head here, but don't sime trains from the Slovenian capital going to Switzerland run up through Lienz Austria and then the Pustertal Valley? I know there are several beautiful bicycle touring routes that run along with train tracks from Slovenia through the Dolomites/Northern Italy. AFAIK (and again I'm just thumbnailing this) Northern Italy is not at all out of the way for your routing, and potentially a more convenient route?

I've ridden a bike over the Alps from Salzburg to Lienz - train links those two cities quickly and easily. And Lienz is 45ish minutes by train to where you would the head up to the Tre Cime for instance?

Anyway the places you mentioned are tied by fast and easy train connections to northern Italy, so don't discount it as a destination if you really want to go.

Personally I think you’re covering too much ground in 9 days, but you have your airfare booked?

How many nights in Europe do you have?

If you prefer quieter over hustle and bustle you may not want to stay IN Innsbruck. While I love the Innsbruck area and its old town, outside the old town the city is hustle and bustle and noise and traffic and the outskirts is quite industrial.

Postojna or Škocjan Cave?

Postojna Cave is a beautiful stalactite cave that is easy to visit for everyone. Crowds of people are taken inside the cave on the cave train.
It is the most beautiful cave I have ever seen. Photography is permitted.
Not far from the cave is Predjama Castle, a cave castle.

Škocjan Cave is a different experience.
The special feature is the large underground canyon through which the river Reka flows.
The first part is the Silent Cave (Tiha jama) with stalactites and stalagmites, which, however, cannot compete with Postojna. At the end of the Silent Cave, you have a great view of the underground canyon, which is crossed on a 50-metre-high bridge. It feels like the bridge in Moria Cave in Lord of the Rings. Finally, you return to the surface via the Great Dolina (Velika dolina). This is where the guided tour ends. Photography is not permitted in the cave and is not possible without a tripod.
There is a shorter and a longer route (through another cave) back to the visitor center. In total it is a hike of 3 or 5 km with many steps. You must therefore be physically fit to visit the cave.