Austria, Slovakia, Slovenia itinerary feedback

Can really only contribute to the Slovenian part of the trip.

I haven't been to Bratislava, but having two nights there since you're right off a trans-Atlantic flight does make sense to me.

I think you may be underestimating the travel time between Bratislava and Bled. You'll have to travel back through Vienna, and the Vienna-Ljubljana trip alone is around 5 hours by bus or longer by train. By the time you add the Bratislava-Vienna leg, you're talking about an all-day trek. I don't know how wedded you are to seeing Bratislava on this trip, but it appears that if you travel from Bratislava you're going to need two nights at Lake Bled. (With that much time and a very early start, I think you might be able to squeeze in Vintgar Gorge.)

It's too bad the trip to Lake Bled will probably be too long to allow you to get by with just one night there. Lake Bled is lovely, but it is not huge. You can easily walk around the entire lake (probably between 1 and 2 hours), just to give you an idea. Mind you, I didn't go up to the castle.

You can easily get to Vintgar Gorge by bus from Bled, but I don't think there are frequent departures, so you'll need to check the bus schedule and re-verify it once you get to Bled. You can even visit the gorge from Ljubljana, but that would mean extra time on the bus and possibly hanging around the Bled bus station for quite a while, waiting. Another possibility is putting together a small group of people headed to the gorge while you wait at the Bled bus station. There's often a taxi hanging around, and as of 2015 the fares were quite reasonable if split 3 or 4 ways. If you might want to do that, don't rush to buy your bus ticket!

Bus from Bled to Ljubljana is easy and reasonably frequent, but there is often a van-taxi hanging around the Bled bus station, trying to put together a group of people wanting to get back to the city. The going rate in 2015 was 7 euros per person. The bus is a pleasant-enough ride and not terribly long, but the taxi is, naturally, faster, and would be worth the few euros extra to many travelers. As with the trip to Vintgar, don't be in a rush to buy the bus ticket. The taxi may not show up too long before the bus is due to depart. Those buses are not always on time, by the way.

Ljubljana is a fun city. There are some markets on weekends, so check into those and try to manage your side-trips so you're in the city on the optimal days. The castle takes a fair amount of time to visit.

Either Postojna or Skocjan would be worthwhile. Postojna is a bit more developed, with colored lights and a little train. Skocjan is somewhat more natural; for many that's a good thing. But I think it is also a bit darker. I was 64 when I visited Skocjan, and when the guide spoke of different colors of formations, they all looked the same to me. I'm guessing you're a lot younger than I am, so that won't be an issue for you.

I believe Postojna is a bit easier to reach via public transportation, but I got to Skocjan by bus followed by some walking (perhaps one mile?). Then I got a bus back to Ljubljana right from the caves, so I guess my issue outbound was that I didn't get up early enough. Definitely check bus schedules once you're in Ljubljana to be sure you have the latest info. They probably vary by day of week. The Ljubljana tourist office is extremely well-informed and very helpful.

Ljubljana to Hallstatt seems to be at least 6-1/2 hours by train, so I think some of your Hallstatt activities will have to be pushed to the following day, making you rather late into Salzburg.

Lake Bled is good for a very full day!
Walk around the lake - 2 hours
Take a boat out to the island church - 1 hour - this can be part of your lake walk-around as the launch is on the east side of the lake.
Walk up to the castle, explore, and walk back down - 90 mins - 2 hours.
Take a cab out to Vintgar Gorge, walk the gorge, cab back - 2 hours. It's very close to Lake Bled...10 minute drive at most.
If the weather is warm there is a really great swim dock on the west side of the lake - it's super modern and designed for kids and adults.
If you spend the night, most of the hotels are fairly faceless buildings with little character.
Right under the castle there is a small corner of the old city...there's a little guesthouse there called The Old Parrish House. It's nothing fancy or luxurious but it has old world character, faces the lake, is on a very old square, is updated and remodeled, and extremely comfortable. Open beam ceilings, huge rooms, great breakfast.
Bled has some amazing restaurants!

Logarska Dolina is well worth the effort but you'll have to go on a guided day tour to be able to see it from Ljubljana as public transport access is horrendous from the western part of Slovenia. I'd be inclined to maybe suggest some time at Lake Bohinj in addition to Lake Bled, but with all the day trips from Ljubljana I don't feel like you would do well by cutting your time there.

If you were to rent a car for the Slovenian part of the trip, though, that would open up a world of possibilities. You could do Logarska dolina on your own on one day, then do another day trip by combining of one of the caves in the morning with an afternoon in Piran, leaving you with time to visit Lake Bohinj or do a nice driving loop through the Julian Alps, over Vrsic Pass and through the Soca Valley. If you do decide on renting a car I can provide you with some scenic route suggestions for your day trips.

Your itinerary is somewhat criss-cross over Austria.

To minimize travel times I would suggest to postpone Bratislava in the beginning and to take the evening flight from Vienna to Ljubljana right away. Depending how long your lay-over is, you can go from Vienna airport to the city (15 minutes train to main train station, 6 minutes subway to St. Stephen's cathedral in the very center).

After having finished Slovenia, take the direct train from Ljubljana to Salzburg (4:25 hours). After having been in Salzburg, go to Hallstatt (either by train [one change], or by bus and train [change in Bad Ischl]).

After having finished Hallstatt, go by train to Vienna for the rest of your stay. Visit Bratislva as a day trip (by train or bus, one hour ride).

Never been to Slovenia so I can't contribute to that portion. I did a Munich to Budapest trip with salzburg, Vienna in between. Budapest easily my favorite part (Budapest and Lisbon are the 2 most beautiful big cities I've been to in europe good historic walking tours, architecture too). with salzburg very nice too. I saw a Budapest to Venice direct flight for cheap on Wednesday Saturdays and Sundays in june. Not sure how easy it is to get from Venice airport to Ljubljana but only like 150 miles. their version of uber probably best option. I liked salzburg very much but you could easily chop a night off that. Spend your 3 nights in Budapest cut out Bratislava and just e2 nights in salzburg. I'd at least explore it if those flights fit your schedule and Venice transfer is doable. Vienna to Budapest train is under 3 hours.
Itinerary looks good though very busy. I'd look into whether you can make bled and\or halstatt long daytrips instead of overnight. Or for bled maybe just an overnight trip in the middle of Ljubljana stay where you leave most of luggage at hotel. If you're not driving that is. If you're driving then the hassle of luggage is minimal.