Maybe irishbabe just brings out the worst in people. I'm not sure that I'd generalise at all, but I've certainly met many sociable folk in Croatia, pleasingly well-informed about European history, world affairs etc, and much into playing traditional music in bars and cafes, once you delve beyond the touristy facade. Irishbabe is spot on, however, about the food, and about the scenery.

If you get anywhere near the Dalmatian coast, jimmyt, Rab is well worth a visit along with, or even ahead of, the other places mentioned.

If you depend on the crossing from Venice, be sure it's running at that time of the year - it's seasonal, and the tourist season proper doesn't extend much either side of June, July and August. Also I believe it's geared for day trips from Porec (on the west coast of Istra) so I'm not sure how easy it is to start from the other end.

Opatija on the other side of Istra is a good bit classier and less touristy than Porec, with a more scenic hinterland. But you'd be going at a good time to see the Porec side at its best, when most of the tourists will have gone, but many bars, restaurants etc will still be open. Rovinj, just down the coast, is is a picturesque town, and Pula has a huge Roman amphitheatre.

As for Rijeka, which is a few miles along from Opatija, michaelr may have seen aspects of it that have eluded me. It is a working port that makes no attempt that I can see to disguise its commercial/industrial ambience.

If you get to Porec, whether from Venice or from Trieste airport, we have a flat nearby that you'd be welcome to doss in for a night or two (4/5 people). Any longer and you'd have to pay the very modest rent.

One last thought. Learn a few Croatian phrases in advance. It's always very much appreciated as hardly anyone attempts to learn their language. It can also be useful, unless you're going to be sticking to tourist haunts. Young Serbs and Croatians are likely to know some English, but relatively few adults speak it. They are much more likely to know Italian and/or German.