Yes, taking a day trip to Montenegro from Dubrovnik is definitely worth it if you're looking to explore the beauty of the surrounding region during your 5 days in Dubrovnik. Montenegro offers stunning landscapes, charming coastal towns and a rich history that complements the experiences you'll have in Dubrovnik. Some popular destinations in Montenegro include Kotor, a UNESCO World Heritage site with its well-preserved medieval Old Town and impressive fortress; Budva, known for its vibrant nightlife and beautiful beaches; and Perast, a picturesque town along the Bay of Kotor. Keep in mind that crossing the border may take some time due to passport controls, so it's advisable to start your day early or join an organized tour to make the most of your day trip.