Itinerary adjustments

If you like museums, definitely add a day to Vienna. Even if you don’t like museums add a day to Vienna. I’d drop a day in Paris or add a day to your trip.

Where are you flying in to and out of? What order is your itinerary? How are you traveling between cities?

Salzburg is not a day trip from Vienna. Your girlfriend needs to assume she’ll return to Europe and see Salzburg when visiting Bavaria in Germany since it’s a two-hour train ride from Munich. A direct train from Vienna to Salzburg takes 2h 30m and then you have to return the same day.
If you can take one night from Paris and add it to Vienna that would be best since Vienna needs a minimum of four nights, not three.

Salzburg as a day trip from Vienna you will spend between 6 and 7 hours traveling hotel back to hotel. In return you will bet 3 hours in Salzburg. If you are going to do it, at least spend the night. Two nights are better.

7 nights in Paris, 4 nights Prague, 3 nights Vienna and 4 nights in
Budapest

I would do your 7 nights n Paris (but 6 wouldnt be the end of the world) + 3 nights in Prague, honestly thats the minimum but not too little for little Prague + 4 nights in Vienna so you can see the museums + 4 nights in Budapest for food, going out, the night life (Budapest is the most fun of your stops).

You are spending a lot to even get to Central Europe, so if you can buy two more nights and do Salzburg. But here, I will totally blow your plans.

Plan A
Paris 6 nights.
Paris fly to Prague for 3 nights
Prague to Cesky Krumlov by Bean Shuttle or CK Shuttle for 1 night
Cesky Krumlov to Salzburg by shuttle (same companies, they are affordable) for 1 night (2 better)
Salzburg to Vienna by train for 4 nights (3 if thats all you can do, but then you take an after lunch train to Budapest so you have the morning in Vienna)
Vienna to Budapest by train for 4 nights.

19 to 20 nights. And really three weeks is 23 nights with the last weekend.

Plan B.
Paris 6 nights
Fly to Prague
Prague 3 nights
Fly to Budapest
Budapest 4 nights
Train to Vienna
Vienna 3 nights
Train to Salzburg
Salzburg 2 nights
Fly home

Total of 18 nights (or 19 with a night added to Vienna)

Thank you for the in depth responses!

Where are you flying in to and out of? What order is your itinerary? How are you traveling between cities?

We will be flying in and out of Phoenix. We will be travelling is Phx to Paris, then flying Paris to Prague, Prague to Vienna by train, Vienna to Budapest by train.

Salzburg as a day trip from Vienna you will spend between 6 and 7 hours traveling hotel back to hotel. In return you will bet 3 hours in Salzburg. If you are going to do it, at least spend the night. Two nights are better.

Im realizing now that Salzburg is too ambitious as a day trip from Vienna. If I were to remove a day I think it would be from Paris, making our Paris stay 6 days, and then add a day to the trip giving us 2 extra nights. I could then spend those 2 nights in Salzburg. If we were to add Salzburg would it make more sense to travel from Prague- Salzburg- Vienna-Budapest or Prague- Vienna- Salzburg-Budapest? Thanks again for the help!

I looked at travel times on the Deutsche Bahn website for a trip from Prague to Budapest with stopovers in Salzburg and Vienna (in either order). It appears traveling first to Salzburg, then to Vienna could save you about an hour, but you are not simply traveling from Prague to Budapest by way of Salzburg and Vienna; you are planning to spend time in all four cities. Therefore, the schedules for the various travel legs may be important. I think you'll find more departures from Prague to Vienna than from Prague to Salzburg. Maybe the Salzburg departures will not be as convenient, timewise. You need to take a look yourself.

There's also the matter of cost. There's more mileage for Prague-Vienna-Salzburg-Budapest, so I'd expect the cost to be somewhat higher. The difference may not be great. I didn't look at costs; you might start at the Austrian Railways website, https://www.oebb.at/en/. However, I think you have multiple options for rail companies, at least for Prague to Salzburg or Vienna and for Salzburg or Vienna to Budapest, and OEBB may be more expensive than the alternatives.

You might be able to get an idea of which rail company is least expensive by checking thetrainline.com, which probably sells tickets for all of them. However, it charges a service fee, and I'm not sure it lists all the departures. For me, it would be a place for preliminary research rather than the place to buy tickets.

Seat61.com is a great resource for information on European trains.

One thing we all overlook at times, but acraven is correct about is that its about more than how long, its also about arrival time.

So let’s look at my idea, and I am looking as I type so I may discover that I was incorrect. In raw time, door to door, the flight from Prague to Budapest is about 3 hours faster than the train. But what about arrival time? (now I look, drum roll ….)

First the train:
I just checked the MAV site because I am familiar with it. 36€ in second class and 56€ in first class. Trip to and from train stations are free because you use public transportation. Add 40 minutes to the arrival time to reach your hotel.

• 07:37 to 14:28 you will leave the hotel about 06:30 and you will get a late afternoon and evening in Budapest
• 09:37 to 16:28 not as much a rush in the morning, you will get an evening in Budapest.

Now the flights
Add 1:15 hour on to the arrival time until you reach your hotel. The Budapest airport express bus to near your hotel 6€ or a taxi will no more than 25€ in Prague and in Budapest.
6 out of 7 days of the week only have one non-stop flight. The price begins about $60 but count on $100 with seats and a carryon or checked bag.

• Monday 08:00 to 09:10 so other than the early start to the day, it
  give you a full day in Budapest. That’s nice.

• Tuesday 07:30 to 08:40 same as Monday, but is it getting to be too
  early a start. You are leaving for the airport at 05:30

• Wednesday 15:50 to 17:00 naaaa, take the train.

• Thursday 07:30 to 08:40 same as Monday OR 21:45 to 22:55 which is a
  consideration if you leave the night before, then you wake the next
  day in your new city.

• Friday 07:25 to 08:35 same as Monday, but is it getting to be too
  early a start. You are leaving for the airport at 5:30 in the
  morning.

• Saturday 08:40 to 09:50 so other than the early start to the day, it
  give you a full day in Budapest.

• Sunday 12:20 to 13:30 sort of marginal. Depends if you want to sit
  in a train for 7 hours or deal with airports.

Other than Wednesday, I would fly. Then Budapest to Vienna is every hour, all day long. 2:40 travel time as cheap as 19€ with a seat. And Vienna to Salzburg is every half hour all day long. 2:30 to 2:50 travel time for about 23€ with a seat.

Okay Now the Prague to Salzburg option.

The Train
Cost is 40€ to 100€ depending on class. To and from the stations, lets say free.

• 06:21 to 11:49 with one change. Not a terrible arrival time, but you are leaving for the train station at 05:30 in the morning so a 04:30 wake up call. Its only a 5:28 ride so that’s good and the transfer time is 24 minutes so that’s good for me.
• 10:21 to 15:49 the same details as above. Sort of a late arrival for getting the most out of the day. You wont be walking the streets before 5pm if that early.

And Salzburg to Vienna is every half hour all day long. 2:30 to 2:50 travel time for about 23€ with a seat.
Then Vienna to Budapest is every hour, all day long. 2:40 travel time as cheap as 19€ with a seat.

My conclusion … which of course is my preferences, and we are all different so there is no real “correct” to any of this. Just a bunch of data you might find helpful. But most days of the week I would fly Prague to Budapest then Vienna then Salzburg but only if I was good with flying home from Munich.

Otherwise bite the bullet on that long haul to Salzburg. Okay, to be honest, I am not a huge Salzburg fan. Not my cup of tea. Glad I saw it but I dont live far so I lost little by the trip. I would spend that time in Melk or Gyor or Pannonhalma or Eger or Szentendre or Krems or Tihany, or Cesky Krumlov or …. (but not Bratislava) ……. Where I didn’t add 5 or 6 hours of journey for a an average experience.