There is enough to see in Slovenia and Croatia alone to fill 3.5 weeks. If that is your focus, I would de-emphasize the other places on your list (much as I loved Krakow and Prague). Otherwise, it seems you are trying to fit in too many places that are far apart.

In Slovenia, consider adding:
- Triglav National Park (stop at the Kobarid museum if you are interested in World War I)
- Piran, a beautiful coastal town
- potentially Ptuj/Maribor in the eastern part of the country

In Croatia, consider adding:
- Plitvice Lakes National Park (close to Zagreb)
- Rovinj
- islands on the Dalmatian coast (Hvar, Korcula, etc.)
- side trip to Mostar (and possibly Sarajevo) in Bosnia and Herzegovina
- side trip from Dubrovnik down to Bay of Kotor in Montenegro

I recommend picking up a Rick Steves Croatia/Slovenia book to help plan most of this.

How are you planning on traveling from country to country and city to city? When you say, for instance, you have 3 days in Prague, it looks like to me that you include the day you arrive and the day you leave. You have added up a total of 24 days bsed on what you say in your post. Is this the total for your trip? Or is your trip longer than 24 days?

IMO, you have too many destinations for 3.5 weeks. You will be doing a lot of checking in and out of hotels, traveling by train, bus, plane to destinations. You won't have a lot of time to actually sightsee in your destinations.

Since Croatia & Slovenia are your main focus, maybe you should limit your trip to these two countries & perhaps one other, maybe Czech Republic. Germany & Poland are too far north from Croatia & Slovenia. Or go to Budapest (Hungary borders Croatia & Slovenia), instead of Prague. This will eliminate a lot of wasteful traveling from destination to destination.

Hope this helps.

What Andrew said. It would make a wonderful trip, and I hope you are renting a car.

Okay I think we will refocus our trip to Croatia and Slovenia. Can you help me out with an Iten?

I've posted my Balkans trip reports here; click on my name then go to the bottom to view them. I didn't do 3.5 weeks of one trip, though - I did one trip mostly focused on Slovenia and one focused on Croatia. Look for other trip reports posted here.

Sometimes it's a challenge to fly directly into/out of the Balkans from the US without multiple plane changes. I'd probably start by figuring that out. One way is to fly into a bigger city like Munich or Venice and train/bus in to Slovenia to start. (You wanted to visit Munich anyway, right?) In 2011, I flew into Munich, trained through Salzburg and Graz, Austria to get to Maribor and then trained west to Maribor and Ljubljana and ended in Italy. In 2009 I started in Italy, worked my way over to Slovenia and Croatia by bus and train and car and flew back to Paris from Split via Croatia Airlines.

Do you have a preferred airline? You might be able to start in one of the other cities you had on your list like Munich and head into the Balkans from there. You can also fly between an entry/exit city via easyJet or Croatia Airlines.

With four people, renting a car might work best. If you can find a way to pick up and drop the car in the same country (Slovenia or Croatia), you'll avoid a big drop charge. Maybe start in Zagreb, drive into Slovenia, back down the Dalmatian coast to Dubrovnik, drop the car there and fly out of there somehow.

Try http://www.bahn.com/i/view/USA/en/index.shtml to check train routings between cities. FYI, there is no train service on the Dalmatian coast in Croatia but there is good bus service in the country, should you choose not to drive.

Off the top of my head, here's a rough car itinerary:

Arrive in Zagreb
Rent a car
Drive to:
Plitvice Lakes National Park
Maribor/Ptuj (optional)
Ljubljana
Bled
Triglav
Piran
Rovinj
Mostar
Dubrovnik
Montenegro (optional)
(drop car in Dubrovnik)
Ferry or bus to Korcula or Hvar
Catamaran to Split
Fly home or fly to staging city.

Trying to do too much - and it doesn;t appear you have left time for getting from one place to another. So what you have as 2 days is really 1 or 1.5 at most - since 2 full days means 3 nights.

I would concentrate on what you really want to see versus spreading yourself too thin.

That sounds like it could work! How is the road and driving situation there? What are some not to miss off the top of your head?
THANKS!!

On the "not to miss" list I would put:
- Dubrovnik
- Plitvice Lakes
- Ljubljana
- Lake Bled

I haven't been to Rovinj/Istria but I hear it's terrific. You could potentially skip Piran (which I visited and loved) because it's smaller than Rovinj but roughly similar, I'm told. Zagreb itself offers few attractions though it does have some champions, and you will be arriving there potentially. There are a number of places that are very worth seeing even if "not to miss" that will be easy to see because you will be basically passing through.

I would recommend seeing at least one of the islands (Hvar or Korcula).

A drive through Triglav National Park/over the Vrsic Pass would take a day (from Bled via Kobarid perhaps to Piran) but in May, I'm not sure the pass will be open yet due to snow, depends on the weather next winter.

Again, I highly recommend getting a Rick Steves Croatia/Slovenia book ASAP if you are serious about this trip. It will answer many of your questions. You can also watch Rick Steves's PBS specials for free on Hulu; there are several covering Slovenia and Croatia, if you want to see what these areas look like. Also read the various trip reports here.

We spent 2 weeks in Croatia and 1 in Slovenia, and wished we had more time in both.
I like Andrew’s itinerary, and his suggestion to rent and drop off the car in the same country. Zagreb to Ljubljana is only about 1 ½ hrs, so not a big issue.

We’ve been to both Rovinj and Piran, and loved them both. But I agree, Rovinj is bigger and has more to offer. I don’t remember a beach in Rovinj, but there was one (mostly rocks, not sand) in Piran. Not sure if that is of any interest to you…but in May, it might not matter.

Plitvice Lakes - spend a night there, so you can be there in the morning before the big tour buses arrive
Rovinj – take day trips around Istria, it is beautiful, and the food delicious.
We loved Hvar and Dubrovnik (with a day trip to Cavtat), and they will not be crowded in May.

Both countries are beautiful, with unspoiled nature and wonderful people. We usually don’t spend much time in cities, so we only spent ½ day in Ljubljana, but a few days would be great, especially in nice weather (it poured when we were there). Here are our pictures, to give you an idea of what’s there:
http://ioana-mark.smugmug.com/Travel/2006-Croatia/1921328_d4mK7p#!i=97200488&k=v4BDthB
http://ioana-mark.smugmug.com/Travel/2012-Czech-Republic-Austria/25154352_xp37gR#!i=2062675900&k=gpSHqrq

I think another “not miss” in Slovenia is Vintgar Goge – spend ½ day there when in Bled, and happy planning.

I would definitely recommend the side trip to Mostar from Dubrovnik or Split and I would suggest spending the night which would give you time to explore in the late afternoon after the tour buses have left and early in the morning before they arrive. We found Dubrovnik very crowded when the cruise ships arrived so be sure to do the walk around the city on the walls early in the day. We did the Eliphite Island day trip and it was a great way to get away from the crowds.