We are uncertain if we want to drive from Rijeka to Plitvice Lakes and then down to Split. Any recommendations on buses as to how to accomplish this? Also from what I am reading, the drop off fees and toll roads make it somewhat expensive to do that distance. there are 2 of us and we are going in Mid Sept...any suggestions?
Do you have a surplus of time on your trip? If not, you may want to reconsider.
Some places are best done with a car. Most of Croatia (away from the coast and ferry routes) is an example. While one can get almost anywhere by bus if you spend enough time, doing what you're suggesting does not lend itself to efficiency on a short trip. The frequency of some public transportation in Croatia also winds down outside of "high season" (generally defined as June-August), so if you do plan on taking the bus, you will want to check timetables carefully.
As long as you are both picking up and returning a rental car in Croatia, I would not expect any significant "drop-off" fee at all. Where you would get smacked with a big drop-off fee is if you rent in one country and drop off in another (but that's not what you are proposing). I do not recall any expensive toll roads in Croatia.
You will most likely pay some sort of one-way drop off fee for a short-term rental in Croatia (less than several days). But driving is really the most practical way to do it. (E.g. I see that with Sixt, you'd pay a one-way fee of $71.87 for a two day rental, in addition to the daily rate.)
You could certainly take a bus from Rijeka (about 4 hours, change in Karlovac) to Plitvice from Rijeka, spend the night say at one of the overpriced but convenient park hotels, and catch the bus on to Split the next afternoon. I don't think the last direct bus leaves too late in the day though; might be a slightly later one with a change in Zadar. But you'd probably have enough time to see Plitvice split up over two days. It's fairly easy to get the gist of the park in 6-7 hours. If you stay at a park hotel, they'll stamp your park tickets from the day before so you won't have to pay again the second day - might make up for the inflated price you pay for the hotel itself.
Thank you both for the great suggestions and the time help me...Maybe we will just keep the car ! I do want to maximize time.