If you have time ( like 3 or 4 days) to come to Cat ba island, you can arrange your time as follows:
Half day to visit Cat ba national park, Hospital cave, Trung Trang cave. Those 3 places are in the same area closed by and you can go by bus/ taxi or motorbike.
Visit Lan Ha bay and Monkey Island. You can rent a boat ( small or big depends on your group size) to cruise Lan Ha bay. You can ask the captain to stop at Van Boi Con beach for swimming, go kayaking at Ba Trai Dao area with sandy beaches and islets and then go back to Monkey Island to take pictures of some monkeys, climb up the mountain to have panorama view of Lan Ha bay or you can swim at Monkey Island beach.
Visit Viet Hai fishing village: there are two ways to come to Viet Hai fishing village, an old village in Cat Ba national park with old houses and different way of life. You can go trekking through the forest, Frog lake to come to Viet Hai village, total time about 6 hours trekking in the forest. Or you can go easier way by cruising in Lan Ha bay from Ben Beoharbour to Viet Haiharbour and then rent a motobike/ bicycle to go 6 km on a good and beautiful road.
Activities: You can go swimming at Cat Co 1,2 beach ( most of local people do in summer time), Tung Thu beach, Monkey Island beach or especially swimming in Lan Ha bay. It is so fantastic to go kayaking in Lan Ha bay as there are not many tourists come and you can feel the nature beauty here very well. Besides, you can go rock climbing in butterfly valley or in some islands.