I went to Medulin, a very little town, near Pula, within the Istria region.
The activities that I became aware of in this region were primarily snorkelling and scuba diving.
There are day trips to awesome waterfalls and the like.
In a larger city I am sure that if you were prepared to pay you could do almost anything you wanted. (A few years ago, that seemed to be the culture)
It may have changed, but I felt like Croatia was primarily just a chillout zone. When I went, it was 45degrees everyday, so no one was keen to do anything other than laze around at the beach. about 10 years ago