Island Hopping Croatia?

Too many options to say much more than peruse a guide book to get started (I like Lonely Planet), but here are a few thoughts to point you in the right direction.
If you have the two full weeks, you can include Rovinj/Istria, but if it's 10, then I'd lean toward sticking to Dalmatia. Flights will be a determining factor. If you can fly into Zagreb and out of Dubrovnik, for example, that is a case for saving Istria for later, and seeing Plitvice and then a couple islands between Split and Dubrovnik.
Start by taking a look at your flight options--that is the stickler for US visitors, and since the entire coastline is beautiful, you really cannot go wrong. I'm a fan of the less mentioned northern islands after four visits, but no doubt it is the southern islands that get all the acclaim. That is just to say that if you are interested in Istria, you could explore in the north and not spread yourselves too thin by trying to get al the way south.
Do you want to drive or use ferries/bus? The latter are perfectly acceptable (comfortable and convenient) for getting from place to place, but for Istria, interior, car is needed. You can always do a day rental if on a large island, and not being tied to a car frees you up to use the catamarans.
Late May is gorgeous--but the sea will still be fairly chilly. I always swam anyway, but it may be a reason to focus on the south.

Terry - Hvar, Brač, and Korčula are must see islands on the Croatian coast.

We are flying into Venice ( a favorite city of ours) and plan spending a couple of days there. From there the plan is to take a ferry over Rovinj or Pula as a starting point.

I could see us renting a car once or twice. Once at the beginning of the trip to explore around the Istria. Then island hop down the coast for a while. We definitely want to see Dubrovnik and also plan on flying out of there so, if island hopping down the entire coast is too much, we might have do the upper coast first. Then drive a portion, maybe to Split, and continue our island hop down to Dubrovnik.

"Hvar, Brač, and Korčula are must see islands on the Croatian coast."

Thanks. It looks like these three islands are between Split and Dubrovik so it should work out well.

Definitely Terry

Yes that sounds like a good plan if coming from Venice. Deciding what to skip is always the hardest part. If you plan on spending time in Split and Dubrovnik, deducting that will determine how much time for how many islands. Once you settle on that, you can decide if keeping the car makes sense or not.