Pula is a beautiful town on the Istria Peninsula near Rijeka, as the biggest city in this region. It is famous for the great Roman amphitheatre- Arena and other remains from that period of history such as a town gate and a triumphal arch.
Nearby is the Bjijuni National Park which is well worth a look. You can get a ferry to the island and get a guided tour around the island.
If you head south to the southern tip of the Istrian peninsula you will find some amazing beaches with pure white pebbles and very clear water.