Central Europe

I'd start by reading guide books to those countries to see what places interested me, and I would soon realize 3 weeks was not long enough to cover all four countries. Much will depend on the breadth and depth of your interests, but 3 weeks isn't a great deal of time for Italy alone, which has a great variety of top sights running the full length of the boot. In fact, Italy has so many options that I'd start my planning with it.

Number 1: get your airfare locked in. Fly into one of your countries and home from another.

Well, I wouldn't rush to buy airline tickets before having a pretty solid itinerary. We've had a few first-timers ask for itinerary advice after buying tickets based on over-optimistc ideas about how much ground they'd be able to cover.

look here on this very website: Explore Europe There are pages of planning tools for every country, e.g., austria planning .

But the best advice you've already received: spend time with a good guidebook and you'll find ideas and questions you didn't know to ask. Rick Steves has made a fortune giving guidance, so its hard to replicate general info in a forum thread.

One way to start is to watch the RS videos for each of those areas to learn a bit about the sights - and then go to the guidebooks.... Also you can check out what tour companies (RS and others) propose to lead tourists through city by city...

Solid beginning steps:

Decide on the length of the trip - 21 days.

Decide on how may days you want to stay in each place - I want at least three nights in each location, maybe two nights in some; 21 days divided by 3 means you can only do 7 places, maybe up to 8 or 9 (not counting the travel time between them, which of course you do have to count).

Decide on your places.

Based on the places you choose - Decide on your preferred mode of travel - train, plane, car.

Decide on the route(s) you want to take using train websites, ViaMichelin website, etc.

Revisit your choice of places when you discover how long in travel time/how difficult in access the places are you have first chosen.

Enjoy the planning process.

Some early comments:
- Italy is fairly large, what interests you most?
- Austria is a bit of an outlier, too, especially the Vienna area: what draws you there?

Otherwise, in three weeks you could land in Zürich, travel from the Berner Oberland, to the Italian Lakes, Florence, then Venice, and take a ferry to Istria, then fly back home from Ljubljana, possibly having visited lake Bled too. On the fast-paced side, but doable.

I would check out Rick's books on those countries from the library, or if you prefer, buy them here: https://store.ricksteves.com/shop/guidebooks His books are SOOOOOOOOO worth the money. Seriously, as compared to many other approaches that some books take, his are much more specific and useful. Instead of trying to cover "every site in Rome" like many books do, giving you a couple of sentences about each one, he picks the top sites in each location and goes into detail.

If I were doing your itinerary, I'd do it in logical travel order--Switzerland, Austria, Italy, Croatia. Or, put Croatia in there after northern Italy, and then end in Rome, which will be much easier/cheaper to get a flight home from (I'm assuming to US?) than from Croatia. Book a multi-itinerary or "open-jaw" ticket into Zurich and out of Rome, instead of a round-trip or two one-ways.

Decide your trip priorities - what do you want to see/do in each location? We can all give you opinions as to how long you should stay in each place, but ultimately that's up to you. Use a site like Rome2Rio.com to see how you can get from place to place (via plane, train, etc.) - I recommend taking trains as often as possible, they are much better in Europe than in the US.

Here's a suggested itinerary - but again, this is just my humble opinion which you can take with a grain of salt. :)

Day 0-1 - fly overnight from ?? to Zurich. Train to Lucerne.
Sleep Lucerne (2 nights).
Train to Berner Oberland. Sleep Murren, Wengen, or Lauterbrunnen (3 nights).
Train (or poss. drive) to Innsbruck. Sleep Innsbruck (1 night).
Train to Salzburg or Hallstadt. Sleep 2 nights. (Alternate here - fly from Zurich to Vienna, skipping Innsbruck, Salzburg, Hallstadt entirely. Or, skip Vienna entirely, as to me, it's your biggest outlier.)
Train (or even poss. boat) to Vienna. Sleep 2 nights.
Drive to Plitvice National Park. Sleep 2 nights.
Drive to Split. Sleep 2 nights.
Drive to Dubrovnik. Sleep 2 nights.
Fly to Venice. Sleep 2 nights.
Train to Florence. Sleep 2 nights.
Train to Rome. Sleep 3 nights.
Fly home.

Total trip length: 21-24 days.

You might also consider dropping one of your four countries for a later date to make the 21 days more attainable.

See these links:

https://www.ricksteves.com/europe/slovenia-croatia-itinerary
https://www.ricksteves.com/europe/switzerland/itinerary
https://www.ricksteves.com/europe/austria/itinerary
https://www.ricksteves.com/europe/italy/itinerary