If you're interested in soft off-road, there are extremely lots of good tracks in Istria, Dalmatia and Lika, though the last region has some drawbacks due to still active mine fields, which are, however, well marked, at least if you're entering from a tarmac road (this goes in part also for some parts of Dalmatia, especially the Zadar county). Soft off-road is easy and straightforward in Croatia, giving lots of joyful moments. If you're planning some hard off-road, you should get in touch with some Croatian club, as you already mentioned. Don't know any, but could find some contacts. If you were interested in Slovenian off-road I could be more helpful.
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