Our family of four spent a lovely day in Kotor in May, 2017. We rented bikes from "Oasis Rent a Car" for 10 Euros/bike. (The office is within walking distance from the port, next to the Galion restaurant). The office clerk gave us a map with suggestions for places to go. We elected to ride along the bay (flat road) to Tivat. The views were absolutely stunning! While there, we stumbled upon a place called Waikiki Beach Resort, with a restaurant and private beach. We had a delicious, inexpensive lunch there and then rented chairs with umbrellas for 12 Euros/person. The beach was very nice, but rocky, with lots of pebbles. (We would recommend swim shoes of some kind.) The bike ride, lunch and beach took about 4 - 5 hours. Afterward, we spent time walking through the old town of Kotor. If you enjoy biking, we highly recommend this adventure!
The bike rental company: www.oasismontenegro.me
The restaurant/beach: www.waikikibeach-tivat.com