Herceg Novi makes a good base for the western Bay of Kotor.
Kotor is the obvious one — 30 minutes by the Kamenari ferry, an hour by road. A full day for the old town, the city walls and the fortress climb.
Perast and the islets can be combined with Kotor or done separately. The boat trip out to Our Lady of the Rocks is the headline.
Dubrovnik is about 90 minutes including the border crossing — feasible as a long day trip but better with an overnight if you can spare it.
The Luštica peninsula — Žanjic, the Blue Cave, the abandoned submarine pens at Rose, the village of Rose itself. Easiest as a boat tour from the Herceg Novi promenade.
Mount Orjen — hiking from Vrbanj village, the dramatic karst landscape, and the highest peaks in the region.
