Hello David,
(Did we exchange messages via CS?)
I also think Istria is a lovely region. It's easily accessible from Zagreb, is beautiful, and there's much to see and do there. You might find some opportunities in Pula, as MalcomS mentioned. You could also look at Rijeka, which is larger. It's perhaps not as scenic, and not technically Istrian, but it's a great hub for going elsewhere (by car, ferry, bus, etc.). I actually quite like it, and it's only about an hour and a half from Zagreb.
If you're more interested in the Dalmatian coast, Split is certainly worth checking out. As the second largest city in Croatia, you'll likely find more opportunities there. It's a gorgeous place, and there's plenty to see and do both in and around the city.
I also really like Zadar. It's smaller, beautiful, and offers easy access to several islands. Zagreb is about 2.5 hours away by car. It's also home to the "Sea Organ," which is pretty unique: http://www.youtube.com/watch?v=qJWldNdG4Lo.
I don't know which of these places would be best for opportunities teaching English, but you could perhaps do a couple of searches on moj-posao.net and see if you come up with anything. (You can type either "engleski" or "english" in the search box for jobs that require English language knowledge.) As with most places, making connections and simply putting yourself out there helps, so I would just start searching for companies and sending out inquiries. I'm actually not legally employed (yet), so perhaps I'm not the best person to advise you, but I did find volunteer work, which in turn helped me get my visa, by simply sending out a letter of interest. I've found that people here are generally very responsive and excited about your interest in their companies.
Hope that helps!
E