Croatia and Slovenia

I am thinking of going to Slovenia and Croatia next year in either May or September. I have a few questions I am hoping people could answer:
First, is it better to go in May or September. I am really trying to avoid the crazy summer crowds, but I would like most places to still be open. I know these months are the bridge months before and after the more crowded summer time, but I was wondering if one would be considered better than the other?

I am going to try and go for 14 days. My plan is to fly into Slovenia and stay for about 3 days. I would like to stay in Ljubljana but my main goal is the Predjama Castle and Postojna caves. Are these sites worth a visit or are they extremely touristy? I don't mind touristy as long as it is not like going to Disneyland.

After a few days in Slovenia, I would like to travel down to Pula and stay there for 4 days or so and visit Istria. I wanted to see all the Roman Ruins in Pula, visit the Basilica in Porec etc... Then I would like to go to Split for about 3 days and then down to Dubrovnik for 3/4 days.
Is this biting off more than I can chew? What is the best way to travel to all of these areas? I was thinking of taking buses and taxi cabs while in Slovenia and then once I reach Pula renting a car to travel throughout Croatia. Is this a good idea? I figured if I rented a car I could get to places much more quickly than buses and trains. But, I have never driven/rented a car in Europe so I don't know if this is a good idea? Is it easy to get from Pula-Split-Dubrovnik without a car?

Thanks for any help you can offer!

Posted by Laura
Rick Steves' Europe
6008 posts

Responses are collecting on your duplicate post under General Europe.