You're smart to want to see something besides the coast. The inland towns usually get far fewer tourists (Ljubljana being a major exception, but it's worth being part of the tourist scrum).
Croatia has some train service (Google Croatia Rail Map) as far south as Split. The line doesn't hug the coast. Buses provide more complete coverage and are comfortable. English is widely spoken, at least by those under 40, so the country is easily managed by solo travelers. Just check outbound bus schedules as soon as you arrive in each town to be sure you know the current situation. You'll be traveling during a period when there could be some schedule changes, and I don't trust online bus schedules to be totally accurate.
Zagreb, well-connected to Ljubljana (Slovenia) by rail and bus, is a city I really enjoyed. It has a large, bi-level historic district and lots of good museums (including a superior though small museum of naïve art and the quirky but fun Museum of Broken Relationships. There's an active café culture that makes it lively, but the vibe is different from Ljubljana's, perhaps because Zagreb's historic district doesn't have a river running through it. If you want to start your trip in Croatia or Slovenia, definitely check out flight options into Zagreb, which may be less expensive than flying into Ljubljana.
There's shuttle service (I think by GoOpti) from Ljubljana to Venice, or you can cobble together a rail, bus or (probably) combo routing.
The Plitvice Lakes National Park is magnificent, but it is critical to spend the night before your visit in or near the park. It gets slammed by day-trippers to the point that the experience is, frankly, far inferior to what it should be. There's bus service to the park from both Zagreb and Split. If you time your arrival for mid-afternoon, you might enjoy a few pleasant hours in the park that day, then finish up early the next morning. I made the mistake in 2015 of not doing what I am recommending, so I can offer no specifics on where you might stay. Booking.com does have a lot of listings in the area, and I think you might find a small place outside the park (and thus less expensive) that offers shuttle transportation; obviously you'd want to check into that carefully. If you can manipulate your schedule so you visit the park mid-week, that might also be helpful. It was my impression that a lot of the park visitors were Croatians.
The Istrian Peninsula is lovely. The coastal towns of Piran (Slovenia), Rovinj, Porec and the tiny Vrsar are worth visiting. Rovinj and Porec have totally different appearances. The one-day bus tour I took through Inland Istria was a very efficient way to see an area that is not well-served by public transportation; it was worth being tied to the tour schedule to be able to get to so many places in one day. Two key stops were the medieval towns of Motovun and Groznjan. That bus tour ran only on Wednesdays in 2015. This seems to be the tour I took; it's quite reasonably priced at 60 euros.
I also liked Zadar. I haven't been to Split or Dubrovnik recently.
How much time will you have for your pre-tour explorations?
Dejan has been very generous in describing what there is to see in Slovenia. It's worth looking for earlier posts in the Slovenia forum. I haven't seen much of that country due to an untimely illness that pinned me down in the lovely Ljubljana for nearly a week on my last visit. The Skocjan (more natural) and Postojna (more touristy, with colored lights) Caves are worthwhile, but I'd just pick one. The Vintgar Gorge is highly developed for tourism but makes for a nice couple of hours or so out in nature. All reachable by public bus but Skocjan may also require walking 1+ miles.