In Croatia with TIME

What about Zagreb and inland region (north)? Although it's also coastal, Istria is probably lesser visited than the middle and southern coastline (northern/southern Dalmatia including Split, Dubrovnik, and several islands).

Yes. I was thinking about renting a car from Zagreb and doing a loop through solvinia through istria back to Zagreb.
Do you think it's feasible to find last minute accommodations? I did a similar adventure in the American Southwest.... I really love planning... Nothing!

Just showing up in town, without even having made a reservation the night before, is somewhat risky. You run the risk of hitting an area when there's a special event going on and other travelers have snagged everything available. Zagreb is great, but it's the capital and the main business center in Croatia, so there's a lot of demand from folks other than tourists. Although May is well before the peak, I've had trouble finding lodging in coastal towns in several countries, even 2 to 4 days in advance. You'd probably be OK most of the time, but not necessarily all the time. I'd consider the just-show-up technique risky in Istria, etc.

There seem to be a lot of small accommodations (private rooms in people's homes, stand-alone apartments, etc.) in Croatia. They're called "sobe". Booking.com has some such options, but quite a lot are just on the books of a local travel agency. You don't want to use your precious vacation time, wandering from agency to agency, looking for a place to sleep. Unless you're both heavy smokers, you also don't want to end up in a room (however nice) in the home of a chain-smoker, as I did in 2015.

Definitely go to Plitvice Lakes National Park. To assure yourself of a decent experience (as opposed to a mob scene on the walkways), spend the night before your visit very near the park and go in at opening time. That will give you a head start on all the day-trippers and the people who like to sleep late.

I did a driving trip in May of 2015 through Croatia and Bosnia (with brief stops in Kotor, Montenegro and in Ljubljana, where I had been before). I too had a loose itinerary, and I booked almost nothing in advance; usually I booked the night or morning before. Sometimes I booked an hour or two before or just showed up at a front desk. I had no trouble at all (one night in Zadar was tricky because there was some sort of race event the night I decided to stop, but somehow I lucked into a great little B&B in the old town and even got a free parking spot! Booked less than 24 hrs in advance, on Booking.com.) So, yes, I'd say if you are flexible and have an internet device to make bookings online, go for it. Plan on having a working mobile phone if possible, too - just makes things easier, even if you can find WiFi pretty easily. (I use my smart phone as GPS with Google Maps - you could use it without phone service using "offline mode" if need be.)

I would research hotels and areas ahead of time, even if you don't nail down specific reservations. Then when you are ready to book a place, you have a few possible hotels already on your list to start with, instead of starting cold.

Be sure for driving in Slovenia you buy a vignette driving sticker (if you rent the car in Croatia) - otherwise, you risk a steep fine for driving without one! Buy one at the border at a gas station.

Go into Bosnia if you have time.

The cities are Mostar and Sarajevo with major towns like Jajce and Travnik that are also quite nice.

We did a trip that included staying in those four places and really loved it.

Kravice Falls, Polace, Blagaj and Medjugorje near Mostar.

Sarajevo is an amazing place; I could live there.

Back in Croatia, in May the Istra peninsula should be very available. I loved the towns of Pula and Porec there. Of course, even though it's hugely popular you should still visit Plitvice Lakes National Park. Helpful Tip for PLNP: Park in the more southern of the two major car parks (across from Plitvice Hotel). If it's crowded there, pass the ticket office there and walk towards and then past the hotels towards the lake. There is a separate ticket office there that never has lines.

I will name few spots I am familiar, but many places to visit.

Korcula island with Marco Polo Museum. Best Korcula wines are Posip and Grk. Having a car you can visit Peljesac and Dingac wine production. All original wines. Local cuisine in Zrnovo or Ston. NP Mljet with the monastery. Korcula actually has many monasteries.

Dubrovnik, I am born in Dubrovnik but just not creative today :).

In addition to this, I will add Plitvice lakes and Nikola Tesla museum. On the way, by car, you can look for Croatian templar castles.