I use this simple two prong adapter in Slovenia, Croatia, and most of the rest of Europe:
www.amazon.com/gp/product/B01C5TG278
But you can buy them at AAA, Fry's (probably), and elsewhere retail locally if need be.
Slovenia is on the Euro, so you need to use Euros there like in France, Italy, etc.
Croatia uses the Kuna (HRK), not the Euro. You may find hotels prices in Euros but you will pay in Kuna. Sometimes places will accept Euros in Croatia but likely at a poor exchange rate for you - better off paying in Kuna if possible. Or use a credit card that has no foreign transaction fee (e.g. the Costco Citi Visa).
I use my ATM card to get cash all over Europe, as much as needed, when I travel, but my credit union travel card charges no fee per use, so I just get 20 Euros here and there at ATMs, and the ATMs in Europe mostly don't charge either (haven't been to Spain yet and I hear some Spanish ATMs charge). I use credit cards whenever possible, though, and limit my use of cash.
You can certainly take a bus between Lake Bled, Ljubljana, and elsewhere. Here is a site with bus schedules for that:
https://www.alpetour.si/en/public-transport/bus-schedule-browser/
However, driving is really easy in Slovenia, and if you don't rent a car, you'll miss some pretty scenic drives - like the Julian Alps loop drive (Vrsic Pass) or the beautiful drive from Skofja Loka to Bled via the mountains through Jamnik and Kropa. The cheapest rental cars in Europe tend to be manual transmission, however, so expect to pay extra for an automatic if you need one (and reserve one in advance). Try EconomyCarRentals or Auto Europe for car rentals. One way rentals are possible (within Slovenia) and you need not go to the airport to rent a car.
You don't want/need a car within Ljubljana itself - there are big pedestrian areas, and if you have a car you'll probably need to park it in a lot, unless your hotel/B&B has parking. It's a terrific walking town.
I looked at your hotel in Google Maps - it's in the south part of the center of town (but it's a tiny town). The cheapest way would be to take a public bus from the airport to the train station then a local bus from there (or a very long walk, which). I have not used the local buses in Ljubljana - I think one is required to buy a e-card (as in most European cities these days) then add funds to it (which you can use to get back to the train station to get to Zagreb later). I'm sure you can find this info by googling. You could also take GoOpti from the airport directly to the hotel.
(I might have picked a hotel on the north side close to the train station to avoid the hassle of getting between the train station and your hotel. Too late to change it up? It's not an impossible walk, but it could be raining when you get to Slovenia - I've had rain there part of every visit!)
It's never occurred to me to worry about Hepatitis after visiting Slovenia and Croatia four times (never gotten a vaccine to travel anywhere, actually); I guess people might worry about it in France or Italy too. Slovenia and Croatia are developed "first world" countries. I do buy bottled water (usually very cheap) in Europe anyway much of the time but have also drunk the tap water in both countries many times.
