We are going on the August Eastern European tour that ends in Lake Bled. I have read that many people then fly out of Ljubljana. Some advice from others that have done it about what airlines and to what major connection cities have been the best for connections back to US. Thanks in advance for any responses.
It will make some difference where you want to return to in the US. Myself, I generally prefer to transfer within Europe than within the US, especially since you have to go through US customs and immigration at your first port of entry, even if connecting further on. Transfer hubs in Europe depend on the airline. From Seattle, for instance, we have direct flights to/from Frankfurt, Paris, Amsterdam, London, or Reykjavik, but some US cities have more options. If you don't have one or two preferred carriers in your area, then start with a wider search through an engine like Skyscanner, Google Flights, Kayak, etc.
I'm flying from Ljubljana to Frankfurt and then home to Canada.
Venice also could work. I have used Venice to fly INTO Ljubljana a few times. Last time for example I got a direct flight from the US to Venice. You can use a transfer service like GoOpti to get you from Lake Bled to Venice Marco Polo if that works for you on the desired travel dates. But if flying out of Ljubljana is just as cheap or just as convenient, I'd certainly do that. Everyone's flight scenario is different!
I don't know where your home airport is but here's what worked very well for us last August after this tour. We flew home (Denver) from Ljubljana on Lufthansa, connecting easily through Munich, then (also on LH) nonstop to Denver. The tour was terrific! Just as an aside, Ljubljana is a wonderful city. We had been there before and loved it so much we added on several days after the tour. Easy taxi from Bled to Ljubljana, which our guide arranged for us and others going our way.
We stayed in Ljubljana for 8 days, but flew in and out of Venice. It's about a 2 hr 15 min ride each way with GoOpti.
We did this tour and chose Lufthansa both ways. For our return trip our guide arranged a midmorning taxi from Bled to LJU and we flew to Munich for our non-stop flight home to Boston. We've done two tours, flying out of and into LJU, and it is a small convenient airport that is close to both Bled and Ljubljana and easy to navigate.
First, we do need to know your US airport(s) if we are to give anything other than general advice.
General advice: I see this tour starts in Prague and ends in Ljubljana. To find flights, use the "multi city" or "multiple destinations" option on Kayak, Google Flights, etc. Put in your home airport to PRG and LJU to your home airport, and see what comes up. The most important thing is to make sure you are all on one ticket, unless you have a day between flights. More and more, the flight searches are showing flights on separate tickets (for instance, one airline from the US to Europe, then Easyjet or Ryanair on a separate booking within Europe). If you are on separate tickets and there are any problem, you are responsible, both financially and logistically, for fixing it. If you are on one ticket and miss the connection because the incoming flight is delayed, the airline is responsible for putting you on the next available flight at no extra charge. Of course, the key word is available - so definitely plan on getting to Prague at least one day before your tour starts, to get over jet lag a bit and allow for delays.
Prague has very few nonstop flights to the US, and Ljubljana has none, so you'll be connecting in Europe. If you have a choice, Munich and Zurich and Amsterdam are generally considered "good" airports to change in, London Heathrow and Paris CDG are generally considered "bad" airports for changing, and others get various reviews. For instance, I've had no problems at Frankfurt, despite its large size, but others have. Again, saying "change in Zurich" won't help you if you can't get a convenient flight from Zurich to your home airport.
In August, Venice has some nonstops back to a few US airports (Newark, JFK, Chicago, Atlanta, Philadelphia). If you want to go to Venice after your tour anyway, this can work well. Another advantage is that these US-bound flights leave Venice around late morning or early afternoon; many of the flights from Ljubljana will leave VERY early, to connect with a US-bound flight from a hub like Frankfurt.
As for airlines, you'll again see good and bad about almost every carrier. One to know about is Turkish Air; they can be much nicer than other airlines, even in coach, and they often have good prices.