Kotor is GORGEOUS and this is coming from a Kiwi (one of the hardest groups of people to impress with natural beauty in the world). Its feels like a Milford Sound-(google it)-esque water inlet with jade coloured water, complete with a fortified old town and overlooking ramparts.
While you may only need a day here to see everything, it will be a very enjoyable day, perusing the endless nooks and crannies of the old town shops that feature numerous locally made products, dining at one of the many delectable (and affordable) restaurants, patting the infamous amount of friendly stray cats (there is even a cat museum highlighting the local personalities) and climbing the ramparts up to the overlooking chapel and fortress.
Climbing the ramparts took about 50mins climbing up (with breaks) and around 20mins back down. It provides the best vistas in the city and made me feel like a teenager playing Age of Empires again.
If you have the time, Kotor is a must see that will leave its beautiful impression on you for years to come.