11 days - Vienna and Budapest - What else?

I would spend at least one more day in Budapest. The spas are worth a day to themselves and the Terror Museum took me an entire morning to enjoy. Bratislava might be worth a short day trip. It's only an hour train ride from Vienna and a round trip ticket costs only around 15 euro.
I am a big noisy city kind of person, so I can ALWAYS find something to do in the big cities. However, I've heard a lot of people enjoy Eger or Estergom in Hungary, or Mauthausen concentration camp near Vienna.

I would give 2 or 3 days for Salzburg. Take one day for a daytrip to the lakes region from there.

I would recommend four full days in Budapest. Sopron, Hungary is a nice stopover on the way from Vienna to Budapest. Eger is a nice top, but not on the way since it's the far side of Budapest. Salzburg is wonderful, but also the wrong direction. Croatia would be way out of our way, although you could fly there it might be pricey and/or inconvenient. In researching Croatia myself, I found that it was not as easy to reach as Budapest, Vienna, Prague. It's probably easier for flights within Europe that flying there directly from home.

Personally, I think that a day trip to Melk Monastery in Austria and a boat trip on the Danube (both can be done in one day) is really an experience of a lifetime.

I'd divide the entire time between the areas around Vienna and Budapest, two big cities. Salzburg, very charming, is 2 1/2 hours from Vienna by fast train--I'd skip it on this trip. Both cities have great public transportation systems; if one gets bored exploring these cities, then one has become tired of life.

You might also consider visiting Pecs from Budapest. It is in the south of the country and works best as an overnight, but is doable as a long day trip.

Lake Balaton is a big resort area not far from Budapest. Also if you like pottery or porcelain, Herend is famous and in that direction. You could also spend a day in Bratislava, it's close.

Melk Abbey is spectacular and Salzburg is one of our favorite spots ever. Charming and scenic.

Croatia is beautiful but out of the way. I agree with everyone else, Bratislava or Pecs, Gyor, or take a car and drive along the Danube to Szentendre, Ves, and Visegard or more time in Budapest.

We were there last summer. Melk-Krems-Danube trip was a very fulfilling day, and lots of fun. Highly recommend. If you have a car, Mauthausen Concentration Camp is 2 hr. W of Vienna - a very good stop, but of course not a fun day - a day of reflection. In Budapest, there are many small side trips. It all depends on the car situation. There is also the Austrian countryside. We were there in June, and went to Croatia - 9 more days than you have. We spent 5 days in Budapest, and 4 would have been enough. The baths are fun. The central market is an absolute wonder. If you like opera, the opera in Budapest is really very inexpensive. See a familiar work, as the opera is probably in Italian and the subtitles in Hungarian.

I would give Salzburg more time also. It is a nice contrast to Vienna. Very beautiful city.

If you only like big cities you could do just Vienna and Budapest, but I prefer the variety that Salzburg or some other smaller village or town has to offer. It will balance out your trip and is such a great walkable city to visit. One of my favorites.

Another vote for Lake Balaton, if you want beautiful scenery. If you'd like to hear some music, then a night or two in Salzburg with a couple of concerts there and in Vienna as well, would be pleasant, The lake district between Vienna and Salzburg also offers beautiful views.

You definitely need to do the Salt Mines in Salzburg...one of the best experience I have ever had. A place called Salz Welton does tours...very awesome. If you're in Vienna you may also consider a day trip to Cesky Krumlov, its just over the border of Czech, it's a very charming city, youtube it. Can definitely be done in a 1 day. If your in Vienna you could tour the United Nations headquarters there..that was pretty awesome if that's up your alley. I really enjoyed that as well. Lake District is a must see...you basically drive through a lot of that on the way to Salzburg if you do the Salt Mines. edit I've been to Mauthausen concentration camp, it carried a lot of weight to it. I don't have any jewish background and still it was very somber there. Good experience, glad I got to see it, but be prepared to not be laughing and joking as you leave ;) As for Croatia, I would skip that and go to Zurich! Haha. Enjoy your trip!