We will be staying in Trogir the nights of June 1,2,3rd and are looking into a ferry trip possibly to the island of Brac. This would only be for a day trip so wondering if it is worthwhile doing. We are starting our trip with 4 nights in Dubrovnik and from there will be spending a day on the island of Lokrum and will also be visiting Kotor and Perast. From Trogir we go to Plitvice for 2 nights, and then Rovinj for four nights before heading into Slovenia and eventually, Venice. We will have a car rental after our time in Trogir.
We haven't planned a trip to an island other than Lokrum and I feel we should be doing this but not sure where to fit it in. Any ideas welcome!!
I know people say that you don't need that much time in Dubrovnik, but we don't get there until late the first night and will be suffering jet lag. I figure the first day we will explore Dubrovnik, the second day we'll go to Lokrum and the third day we have a tour planned to Montenegro.
I thought about taking a ferry from Dubrovnik to Split, but have also heard that the bus trip from Dubrovnik to Split is wonderful so that is what we decided on.
Still in the back of my mind that we should be seeing one of the islands. We get to Trogir late in the afternoon on June 1st and will be leaving on the morning of June 4th so only have 2 days to explore the Trogir and Split area. Appreciate any advice you can give me.

Should be a great trip!

I didn’t go to Brac, and so can’t comment on it, but the islands are, IME, quite different from one another. Lokrum is charming – but it’s TINY. It has remnants of a monastery and one-time archduke’s estate, some lovely gardens, and … uh … I think that’s about it. In contrast, Hvar and Korcula both have several cities, so quite a different thing.

What is that you think you’ll be missing if you don’t go to an island?

And how willing are you to skip the opportunities in and around Split / Trogir in order to make time for another island?

I think it's just because everyone talks about the wonderful islands that I think I might be missing something important!! I do want to spend time in Trogir and Split so probably trying to fit too much in. Maybe we'll go to one of the small islands off Trogir or maybe I'll just leave things the way they are!!!!

The islands are amazing -- so are the other places you'll visit. No bad choices, IMO, just lots of very difficult ones!

Sounds like you can wait until you're in Trogir to decide....

And of course, some other Fodorites might have opinions or experiences that lead you to a different opinion.

Hvar and Pug are actually really worth visiting, but you have to take a ferry

Lokrum has nice views and would be a nice place to swim. It is mostly a respite from the crowds in Dubrovnik. An alternative might be to do one or the organized tours of the Elafiti islands (if it's really hot and you want to swim, for example).
When you get to Split, store your bags (near the bus station there is a "garderoba") and walk around to get a taste--then you will know if you want to return from Trogir to see more the next day. Trogir is truly tiny, so you won't need much time to explore it.
If you are still hankering for an island, you can return to Split in the morning and take a boat to one of the islands (Brac or Hvar).
If you just want to chill, there are two small islands that ferries depart to directly from Trogir (Drvenik Mali and Drvenik Veli). We spent a really lovely day walking across one of them several years ago, there are numerous coves and rocky/pebbly beaches.
Basically, you have options--should be a wonderful trip! If you fall in love as I did, you can return for an all-island trip.