In my opinion, the most interesting cave is San Cassiano/Skocian in Slovenia, not far away however.
May be you wil like the scenic coastal towns of Porec/Paarenzo, Rivinj/Rovigno and Piran.
Kvarner Island ferries:
Brestova (Istria) - Porozina (Cres Island):http://www.jadrolinija.hr/pdfs/334%20zima%202013.e.pdf
Merag (Cres Island) - Valbiska (Krk Island): http://www.jadrolinija.hr/pdfs/332%20zima%202013.e.pdf
Valbiska (Krk Island) - Lopar (Rab Island): http://www.lnp.hr/en/timetable

neckervd, thank you much for the car ferry schedules. Porec and Rovinj are on the list of cities to consider exploring. What would we see in Piran?

four,
We visited Croatia (and Istria) in 2006 and Slovenia (and Piran) last year; we loved them both, although they are somewhat similar. Here are my pictures (labeled), to give you some ideas:
Croatia http://ioana-mark.smugmug.com/Travel/2006-Croatia/1921328_d4mK7p#!i=97200488&k=v4BDthB
Slovenia http://ioana-mark.smugmug.com/Travel/2012-Czech-Republic-Austria/25154352_xp37gR#!i=2062675900&k=gpSHqrq

Sounds like a great trip that will cover some of the less-touristed parts of Istria and the Kvarner Gulf region. I'll look forward to hearing about your adventures!

Re: Rijeka. I didn't sop in Rijeka, but if you are going that far, you might consider walking the Lungamare from Lovran to Opatija. It is lovely!

Re: Senj. -- I stopped at Nehaj Fortress a few years ago because it was a convenient break in an otherwise long-ish drive. I found it mildly interesting, but I'm not sure I would have gone out of my way to see it.

Re: the Kvarner Islands -- I enjoyed a night and better part of a day in Rab Town. It's small, but has some very special features.

Re: Gračišće -- I loved Gračišće! I'm sure it's been developed a bit since I was there (in 2009, when it was barely prepared to host tourists, but was moving in that direction, with a few structures under renovation). If the Konoba Marino is still there, do consider trying the home-made pasta with wild boar sauce. It makes me hungry just to think of it! And if Svetlana is still there, please tell her that the woman whose car broke down while there sends her fond regards. (I hope she doesn't have to deal with that more than once every few years!) I don't think I've seen anyone else on Fodor's ever mention Gračišće - if you go, please post with your reactions!

Re: Porec -- the mosaics of the basilica are very special and well worth a visit.

FWIW, my "trip report" (just restaurant notes) from the area includes restaurants in Rab, Lovran, Rovinj, and Gračišće - just click on my name to find it.

Enjoy!

xyz, all of your photos are lovely. The photos of Istria are exactly the scenes I am hoping we'll experience (fingers crossed for decent weather!)

kja, wonderful comments, thank you much. Opatija is on our list, annotated with the walk you've described. It looks like dogs are welcome on the walk? (Ours will be traveling with us, and we like to take him with us on outings wherever possible.) As for the Kvarner Islands, Cres sounds the most interesting; we'll see what happens. I've also copied your restaurant notes into my planning notebook; a big part of traveling for us is often eating, and sometimes planning that is harder than the actual travel logistics. Should we stop at the Konobo Marino, I will give Svetlana your regards.

> Opatija is on our list

Actually, Opatija was my least favorite part of the Lungomare -- masses of resorts with lively/loud bars and overpriced restaurants, etc. (The Villa Angiolina was nice, though.)

I don't remember seeing any dogs while on that walk, so I can't say for sure whether they are allowed. I bet they'd enjoy it, though!

We loved Rovinj as a base in Istria. From there we took day trips one day to Pula, one day to Porec and had 2 days just wondering around Istria.
Weather should be great in the summer.

Oh, and food is Istria is soooo good

We took a day trip to Cres Island from Rovinj and really enjoyed it. Got a later start than we'd hoped and had to rush our visit--a few more hours would have been nice. Terrific lunch at Bukaleta! Some of the driving down near Lubenice was, uh, challenging! Great weather really contributed to the overall experience.
http://onelittleworld.zenfolio.com/p390799187

We also visited Groznjan, which is nice. Not sure I'd go way out of my way to visit, but it made a nice combo with Motovun:
http://onelittleworld.zenfolio.com/p499326334

I didn't find Vodjnan to be that creepy--it was more or less what I expected it to be. They only let you stay in the room with the remains for about 15-20 minutes. I haven't been to Kutna Hora though, so no basis for comparison...

Have fun!

Thank you! All of this input is making us anxious for our departure! Especially for the food!

What time of year were all of you thoughtful posters visiting Istria? We'll be there the first week of April, including Easter Sunday. We have to work around the school holidays, and although we're not complaining, nice weather is always preferable.

xyz, Pula is a definite "on the list." Porec is a "perhaps," for the cave.

ms_go, thank you for sharing the photos. I did catch the "dogs not allowed" sign in your Groznjan photos; that's good to know if we decide to go there, as our Foxhound will be traveling with us (he's not exactly a tote puppy!). Following our visit to Kutna Hora, the children (11,16) did not even want to eat lunch in the town, so we headed elsewhere. Maybe we'll pass on Vodjnan. Any more insights on the driving near Lubenice to share? We've driven in Washington, DC, Italy, Germany, France, and Austria, so I'm hoping we're prepared for anything?

We were there late September, and had great weather.

I do not know what Porec cave you have in mind, but we loved the Euphrasian Basilica and its mosaics:
http://en.wikipedia.org/wiki/Euphrasian_Basilica

We drove, and found driving in Croatia (and Slovenia, too) easy, and the roads very good. You need a vignette in Slovenia, but not in Croatia. As far as I remember, we paid all talls with a credit card.

have a great trip!