Day trip from Split: Trogir or Sibenik

We liked Trogir a lot and I agree--it's closer which makes it an easier daytrip.

On the other hand, Plitvice Lakes is quite a distance (4 plus-hours on the bus) for a daytrip. I'd be inclined to spend a relaxed day in Split rather than so much time on the bus.

We did not stop in Sibenik.

Trogir is very nice for a day trip and is not that far away. Sibenik is quite a bit farther away and would require much more travel time there and back.

I love the Plitvice Lakes National Park; have been there three times. But it was a madhouse on my last visit, which was a weekend day in late August 2015. I waited in line for an hour to buy my entry ticket, and the walkways were mobbed. Try to avoid holidays and weekends when scheduling Plitvice. I've always visited Plitvice from Zagreb and am not sure how early it's practical to get there from Split. (Are you taking a day tour rather than a public bus?) Earlier is better.

I have been to both Trogir and Sibenik. I thought Sibenik was a real gem, a real surprise. The approach to the cathedral is amazing as the town clings to a hillside. Once you get up, be sure to visit the medieval Mediterranean garden - what a lovely place. Wander the streets, find hidden squares for coffee. Trogir was also beautiful, but small. Lots of little lanes, but repetitive, in my opinion. A nice place to stop for lunch perhaps.

On the rest of your agenda, I suggest that you drop Plitvice. It is just too far for one day. Krka will give you a similar experience anyway. You could probably even combine Krka and Sibenik/Trogir in one day. For your extra day, I'd highly recommend taking the ferry over to Brac. Rent a car that day, and drive around the island, maybe to Bol. I know you will have done Hvar, but this different again.

Just wanted to thank everybody for the replies. I was aware that Plitvice was a long ride, and I was planning on taking a tour van. It just seemed so spectacular that I did not want to be so close and miss it. But I am now rethinking that. If Krka offers a similar experience, that might be enough. I appreciate all of the thoughts on Trogir and Sibenik. That might eventually come down to a coin flip, or whichever bus schedule is most convenient on the day that we wish to go. I am trying to leave some of the trip open to serendipity. And I will also look into Brac Island. I know that the sandbar beach does look spectacular. Thank you again, everybody.

I'm inclined to agree with Emily and one or two others. Even though I've not been to Plitvice, the vast throngs of visitors deters me somewhat. I can vouch for the beauty of Krka national park, however, especially if you approach by boat from Skradin.
Plus it tends to be far less crowded than Plitvice, which suits me fine.
Then you could possibly make time for a day each in Trogir and Šibenik, as both are totally worth a visit. (You could tie a Šibenik visit in with your Krka trip - they're only about 15 km apart, but something would have to give time-wise). Plus the whole of Trogir town, on the coffin shaped island and Šibenik St James cathedral are UNESCO heritage sites, so with those and the Diocletian's palace in Split, you'll have seen three of the seven in Croatia!