Zagreb (Zagreba), the capital of Croatia, is not generally regarded as a tourist destination. Most people planning a trip to Croatia head directly for the coast, unfortunately, leaving little time if any to see Zagreb.
Croatia's Catholic heritage can be seen in its celebration of Christmas, which falls on December 25 in Croatia. If you're in Croatia's capital city, do pay a visit to the Zagreb Christmas market, which appears, along with festive decorations, on the main square.
Croatia's rivers belong to the Adriatic and the Black Sea basin. The rivers in the interior are large and calmer (Sava, Drava, Danube).