Slovenia and Vienna (?) in October

My questions: What's the best way to travel to either Ljubljana or
Vienna from London: train or plane?

Define best please. It is a long trip by train, but pretty easy and can be fun. Especially if you add stops on the way.

I think we'll drive between the Ljubjana and Vienna if that's an easy
car trip.

It is an easy train trip as well, that will save you the one way car rental.

How many days should we allow for each city and the surrounding areas?
What are the must-sees?

What are you interested in?

Dropping rental cars off in a different country incurs large drop off fees. Take the train.
Our kids have been to Slovenia and raved about their experiences. They said it’s Switzerland at one third the price.

Hi Susan. I'd start my planning for Slovenia by getting the Rick Steves Croatia/Slovenia book unless you already have it. (The old book should suffice for basic planning.) I love Slovenia but not sure what is of interest to you. Ljubljana is not a big city, and there is not a lot to "see" there - no real world-class museums or anything. But it's a charming, picturesque little city I love returning to. It's a great place to come back to at night.

Many people see Lake Bled. More see the natural areas around Lake Bohinj, Trglav National Park, one of cave complexes an hour or two from Ljubljana, etc. If you drive to Bled from Ljubljana, I recommend detouring through the little town of Skofja Loka (worth a stop), then driving the scenic route to Bled through the mountains over the twisty mountain roads through Dražgoše, Jamnik, and Kropa - breathtaking scenery down below! (Roads are narrow but if you take your time it's perfectly safe.)

You could also see Slovenia's tiny little coastline. The old Venetian town of Piran is delightful. There are also towns in east like Ptuj and Maribor that are worth a look if you have time.

Driving in Slovenia is easy, but if you rent a car in Slovenia and drop it in Vienna, you'll likely pay a steep one-way drop fee. One option might be starting and ending the trip in Austria so you would return the car in the same country. (There's more to Austria than Vienna obviously.) But if you just want to see Austria, maybe fly from Tampa to Ljubljana to start, then drop your Slovenian car locally and fly to Vienna or take the train from Slovenia (If you can get a direct flight between Ljubljana and Vienna, that might be easiest, but see how long the train would take.)

You could certainly fly round-trip to London and from London fly on to Ljubljana. It might make more sense to fly from Tampa to Ljubljana (or nearby Venice) and fly home from London on an "open jaw" ticket, then fly one-way from Vienna to London. That saves you the connection between London and Ljubljana which may be unnecessary - not sure. There's a good Slovenian shuttle service called GoOpti that can get you from Venice to Ljubljana in less than three hours. There are also some buses that take you to Ljubljana directly from Venice Marco Polo Airport. Flying to Zagreb, Croatia might work out better - you can take the train from there to Ljubljana.

What's the best way to travel to either Ljubljana or Vienna from London: train or plane?

"Best" is always subjective. For most people, I think the answer to your question would clearly be flying. It'll be cheaper, faster, and simpler. Unless you have an excess of time and are happy to burn both money and time needlessly, I'd just take the plane.

I think we'll drive between the Ljubjana and Vienna if that's an easy car trip.

It would be a easy drive. That said, you might want to consider the following:

• Are you planning to rent a car? Expect a surprisingly high "fee" for renting a car in one country and dropping it off in another (one-way). If you were planning to drive back into the country you rented from and return it there, not an issue. But...
• In most large European cities (and I'd count Vienna as one), having a car is often an expensive, unnecessary headache (parking costs, hassles of driving and navigating in the center of a large city). Ljubljana is a more managable-sized city, but it's still a city, and it's old historic center is not an awesome place to have a car, either. Parking may be an issue in both cities, depending on where you're staying. Not necessarily a deal breaker, but I always think twice before planning to have a car in any big city.
• Check into any country-specific requirements (eg needing a "vignette" for driving in Austria if you rent in Slovenia).

How many days should we allow for each city and the surrounding areas?

Depends on how long your trip is, your tastes, travel style, and what you want to do, of course.

"October" can be a tricky time in mountainous places in central Europe. When in October? On October 1, you'll have a better chance of good weather. I'd worry a lot more about snow if I was going to be there on October 31st. FWIW I drove around Slovenia in May, and our plans were interrupted by a significant snowstorm in the mountains. Anytime in October, you could get snow - of course, more likely later in in the month, but plan accordingly and be ready. Those are serious mountains.

Slovenia is lovely. Have a great trip.

But if you just want to see Austria, maybe fly from Tampa to Ljubljana
to start, then drop your Slovenian car locally and fly to Vienna or
take the train from Slovenia (If you can get a direct flight between
Ljubljana and Vienna, that might be easiest, but see how long the
train would take.)

Good luck finding a direct flight from Ljubljana to Vienna!

Ljubljana is a more managable-sized city, but it's still a city, and
it's old historic center is not an awesome place to have a car,
either.

That's a bit of an understatement. Cars are banned in central Ljubljana.

I made the flight from London to Vienna - I think its 3 hours. Vienna is one of the most wonderful capitols in all of Europe - history, museums, palaces, food, etc.

I know you can go to Graz then to Ljubljana because we made that trip (Actually Ljubljana, Lake Bled, Klagenfurt, Graz, Vienna). But I think you can Vienna to Salzburg then down to Ljubljana too. Maybe make a loop of it.

What we actually did in two weeks was more adventurous. Vienna > Salzburg > Zell Am See > Innsbruck > Bolazno/Merano Italy and Dolomites > Verona > Venice > Priest > Ljubljana > Bled > Klagenfurt > Graz > Vienna. We loved Bolzano and the Dolomites. And you could cut out some of the smaller cities in that loop.

Ljubljana is a nice place with an imposing castle, an interesting town town with three bridges at the same place, and some nice restaurants. But it's kind of vanilla in the sense it does not have any obvious cultural touchstone - unlike France, Italy, Spain, Germany, and Austria. It was kinda Europe-light. It was a bit more economical but not dirt cheap.