It also depends on how you define Eastern Europe and what mode of travel you are using. If you go by car, you can hit smaller towns and you have a lot more flexibility, but you will pay more. I don't find the trains in the former Warsaw Pact countries to be as good or as fast as those in Western Europe. But take that opinion for what it's worth, as I do about 85-90% of my travel in Europe by car. I find the difficulty of parking to be wayyy overstated.

- Vienna (yep, further east than Prague): Vienna is an absolute gem. Smaller than Budapest and not as much fun, but you can't miss it. Staying in Vienna is more reasonable than one would think, you just have to know where to stay. I have stayed in the Marriott Courtyard in Vienna with my wife and kids for less than $80 night right next to Schonbrunn palace and right down the street from a metro stop. You can also use Airbnb or Vrbo. I try and check out everything as you never know what you'll find in Vienna.

- Prague: Everybody goes to Prague, because it is awesome. It really it. Per square km, it has so much to see and has a very party atmosphere to it (unlike Vienna). But everybody also goes to Prague because of how far west it is making it super easy to get to. So that means it can get ridiculously overrun by your fellow tourists. It also doesn't help that Prague's primary tourist area (from Hradcany and the castle, across Charles' Bridge and into Old Town) is in a very geographically compact area when compared to other cities where the tourists tend to get more spread out. Prague can very easily feel as shoulder to shoulder as Venice during Carnival.

Smaller cities:

- Melk, Austria (also, further east than Prague): The Benedictine Abbey is amazing, as is the sight location on the Danube. One of my funnest memories of Europe was taking my 10 year old daughter out at about 10pm to photograph the abbey at night. She says that is where she got her photography hobby. Stayed here, and for Austria the prices were reasonable.

- Cesky Krumlov, Czech Republic: Such a cool place. Always happy to go back. Super fun just to wander the city and check out the shops. The castle is great and punches well above its weight given the size of the city. You won't avoid rip offs here, or high prices though.

- Danube Bend area of Hungary (Esztergom and Visegrad): I love this area and it never disappoints. Fun to go to Visegrad when they are having some sort of Medieval or Renaissance festival. The cathedral at Esztergom has commanding views up and down the Danube and into Slovakia. Cross into Sturovo for amazing views of the cathedral at Esztergom. Sturovo itself is forgettable. Some decent hotels/pensions with great food and reasonable prices.

- Sighisoara, Romania: Birthplace of Vlad the Impaler. But aside from that, just an amazing Saxon city with a great citadel. Was way more cool in the 90's, but I still enjoy it. You do want to stay there overnight as the city has a very interesting mystique to it after dark when things are quiet. Did Halloween there one year before it was a Halloween tourist destination. Pretty fun. Brasov is in the neighborhood as well, as is Hunedoara with one of the best castles in Eastern Europe. Not expensive, but not five star either.

- Bratislava, Slovakia (not much for a capital city, but better than most smaller cities): Worth a stop if you are in the neighborhood. It is the after all the main city in Slovakia. Nice walking street and decent hike to the castle for great views. Some good deals on lodging can be had here.

- Lots of good deals to be had in Budapest. Just watch for them. Apartments, hotels, etc. Budapest is bigger than either Prague or Vienna. The dollar still goes a long way there. Love it there, and you can easily spend three days. Castle, City Park, Parliament, Jewish Quarter, Szentendre, St. Stephen's, Holocaust Museum, Margaret Island, etc.