One Year Anniversary Trip

If you are flying in from the US or Canada you are likely to be sleepy and jetlagged on that first day, so keep that in mind. Zagreb has a really nice bi-level historic district to explore. That would keep you outside during much of your first day and help with jetlag recovery. The lower part of the old town is very lively; that's where most of the restaurants and cafes are. The upper part is quieter but very handsome.

Plitvice Lakes National Park is a wonder. But not if you try to see it on a day-trip. It gets unbelievably blitzed by day-trippers and the walkways are packed. The crowding is severe enough to really diminish your enjoyment of the park, to the point that I'd tell you not to go if you can't spend the night. If you spend a night near the park, you can visit it first thing the next morning and/or late in the afternoon. You can do this on your way from Zagreb toward Split/Dubrovnik. You can find lodgings in or near the park on booking.com. I'm not sure whether you plan to rent a car or use trains/buses, but there is rather frequent bus service from Zagreb to Plitvice as well as service from Plitvice on toward Zadar/Split.

Posters here have mentioned that hotels on the islands commonly require a 3-night stay, but I have not personally verified that. I do think there should be a way for you to make a day-trip to an island and fit in a swim. I'm sure someone else can provide suggestions on where and how that can be done.

The big deal in Split is Diocletian's Palace.

You say you want relaxation, but then are trying to squeeze a 2 week itinerary into 8 nights! Something will have to give.

I assume you are going to be hiring a car, otherwise you are going to struggle. Local schools will be on their summer holidays, so Plitvice and beaches will be busy.

I agree with the previous post that you need a night at Plitvice or drop it. I think with the amount of ground that you have to cover, that you need to seriously consider dropping it. Flying into Split would have made more sense than Zagreb. However, with jetlag, you need 2 nights in Zagreb, then hire a car.

Drive to Split and stay 3 nights. Split needs a day, as does nearby Trogir. Take a day trip to Brac for a beach or Stari Grad on Hvar (better for nearby beaches than Hvar town). Most places on the islands have a 3 night minimum booking.

Drive down the coast to Dubrovnik and drop the car, as you don't need one there. Have 3 nights here. There are several options for a day trip, either Mljet island or Mostar.

I would stay a night in Zagreb, stop in Plitvice for a night (so you can enjoy the park in evening and morning), then head to Split (you will have to choose how much time you wish to spend there. The palace is amazing, but can be seen in an afternoon. There is about as much to do there as Zagreb I suppose, plus day trip options), then go to Hvar or Korcula for at least three nights, then finish in Dubrovnik (it's gorgeous but tiny, unless you have day trips planned you don't need a lot of time there). You can do all that with bus, catamaran, and ferry, so it's purely up to you if you want to rent a car or not (it would be slightly more convenient for the Plitvice stretch but not necessary. One-day rentals are also nice to do on an island for exploring). This is rushed, so make sure you stay one place for three nights so you can relax.
The beaches you could reach from Split and Dubrovnik are not as nice as those you could get to on Hvar or Korcula.
Happy Anniversary!