If you can arrange a day tour of Krka National Park (not to be confused with the completely separate island called Krk, which is nowhere near the park on the mainland), it's definitely worth doing. While it can be done from Split via public transportation, the timing might be a challenge given the limited time on land (nine hours is not really a lot of time once you figure out how to get there and back, travel time to and from, waiting for the return, etc.) Krka park is very spread out, though. There are sites to see among the park that an organized tour could hit in an efficient way. I had my own car and saw only the big magnificent Skradinski Buk waterfall. (You can walk up and around it and even swim below it if you want to - in summer some people will.) You could rent a car for the day and see/do it yourselves, if you are experienced in driving in foreign countries. Driving in Split itself can be challenging, but outside of the city it's very easy.
If you choose not to visit the park, you could of course explore the old city of Split, which is built among the ruins of the sprawling Diocletian's Palace. There are museums (didn't see them) you can simply walk through it (modern shops and stores populate much of the palace). You could walk up Marjan Hill for a nice view down on the city. You could take a ferry (or bus) over to the old city of Trogir, which is more compact and charming than Split (but less interesting and more touristy).
Rijeka is an old industrial port city - generally not of big interest to tourists. The more popular tourist town nearby is old Austrian spa town of Opatija (it's OK - wasn't a favorite of mine, a bit glitzy). You can walk from Opatija to several seaside towns along the Lungomare, a long seaside promenade that passes a lot of old Austrian villas.
If you want to see something inland, a lot of people enjoy exploring the hills towns of nearby Istria. You'd need to get an organized tour for this or rent a car and do it yourself (again, if you are experienced in driving in foreign countries, this is pretty easy to do). Or you could venture over to Pula, another big port city but with a lot of old ancient Roman ruins (I wasn't that excited about Pula; I'd seen my share of Roman ruins in Italy.). The old Venetian port town of Rovinj north of Pula is lovely and charming, worth a stop, about an hour from Rijeka by car. It might be possible in a long day to see both Pula and Rovinj or Rovinj and a couple of hill towns with a rental car.