Slovenia trip suggestions

Buses from Ljubljana are inexpensive and frequent enough that they'll probably work for you, though they aren't always on time. I think you can also get to Vintgar Gorge and to Lake Bohinj from Bled.

Howver, there's usually a taxi hanging out across from the Bled bus station, trying to get enough people going to Ljubljana (about 7€ per person in 2015) to make the trip worthwhile. If you think you might want to take advantage of that, do not rush to buy your Bled-Ljubljana bus ticket.

If you run into some other people heading in the same direction, a shared taxi might be surprisingly affordable.

Public buses also run to Postojna Caves and will at least get you within walking distance of Skocjan. Postojna is more developed (has some colored lights, etc.). Skocjan is more natural.

You can also go to Piran by bus from Ljubljana.

I'm sorry that I have no useful lodging suggestion for you. I'd look at booking.com to get a sense of hotel rates. Then check to see whether there are local hostels with single rooms. Whether there are people renting pricate rooms or apartments (beyond what might be listed on booking.com), I do not know. That was often an option in Croatia in 2015. You can also see what Airbnb has listed.

I don't know what sort of weather you can expect in late September. I sugest looking at the historical hour-by hour temperature on wunderground.com. Go back several years. I suspect hostels may not have a/c (I could be wrong), but it's often available in inexpensive lodgings listed on booking.com.

I'm assuming you are planning to do some day trips from Ljubljana, right? It's a very nice city, but it's rather small and can be seen in a day. I sadly did not have time allocated for day trips, but I think you can do the caves and castle from there. In Bled, I loved Vintgar Gorge (rode an ebike there and back from Bled) and heard great things about Lake Bohinj though I personally didn't make it there. One of the highlights of my 4 days in Slovenia was a drive through the Julian Alps, which I used to connect Bled, where I started, to Ljubljana. I think you can probably do it in reverse or on your way to Piran, though if you don't want a car maybe not. I think there are some tour companies that do the drive round trip from Bled, which you could do on one of your days there. Enjoy your trip - I loved Slovenia and will definitely go back to see all the things I missed.

ETA - I stayed at B&B Petra Varl in Ljubljana for around 60 euro per night, and Pension Mayer in Bled (also about the same price). Both were great, but I really loved Pension Mayer in Bled - the family is super friendly and the grounds of the pension are really pretty. It feels very Alpine.

Three nights in Ljubljana and Bled is good. Ljubljana is lovely, and Bled is a good base for exploring the area.

I loved Piran - beautiful coastal town - but three nights there seems like a lot, unless you want to relax. (You can still swim at the end of September. I did.) You can day trip to other coastal towns I guess.

I've been to Ljubljana a few times. If you really want to save money, you can stay in hostels. I stayed a place last month called B&B Sincere, some private rooms run by the nearby H2O Hostel. The price was good (40 Euros/night for a single room with private bathroom), location was really good - right by the Dragon Bridge - and the rooms were clean though not particularly nice. A bit loud at times - pedestrians walking by at night on the street could be loud. But if you can put up with that, it's a great deal for the location.

Otherwise, look on AirBnB for even cheaper shared housing. I use Booking.com a lot. (If you book B&B Sincere, email them directly, you can reserve even without a credit card).

From Ljubljana, some day trips by public transit: Skofja Loka (by bus), lovely town. Kamnik (by train or bus) - another lovely town.

But unless you have a good reason not to rent a car, I urge you to reconsider. Renting a car isn't required to get between Ljubljana, Bled, and Piran and the attractions - all easy to reach by public transportation. But you can see more with a car, especially beautiful scenic drives. One drive Rick Steves covers in his Croatia/Slovenia book is the "Julian Alps Loop," a drive through the mountains up a twisty old road - really lovely experience, great scenery. I found the scenic drive from Skofja Loka to Bled up through the mountains via Jamnik and Kropa really stunning and memorable. Renting a car is probably not expensive in Slovenia, and driving is generally easy for Americans. If you can't drive a manual shift 5-speed, you would have to pay a little more for an automatic, however.

You could pick up the car as you leave Ljubljana and drop it near Piran - that's what I did on one trip.

Thanks for the tips, folks. I'll keep these in mind as my plans firm up.