Cat Ba Island is the biggest and the most tourist island in Halong bay. It’s more than 350 square kilometres  included Cat Ba town and most of area is national park.
Cat Ba Island is the launching pad for exploring the limestone islets and towers of Lan Ha Bay. Geologically an extension of Halong Bay, and just as dramatic, Lan Ha Bay sees much fewer visitors. But Cat Ba has more to offer than a beautiful seascape to goggle at. This is the northeast’s adventure capital – whether you want to climb it, bike it, hike it or kayak it, Cat Ba has the activity for you.
Almost half of the island’s 354 sq km is protected as a national park, which is home to most of the world’s remaining golden-headed langur population (the world’s most endangered primate), while the island’s cliffs have become one of southeast Asia’s top learn-to-climb destinations.

To visit Cat Ba island you have 2 options, the first is doing it by yourself. It’s easy to take a tourist bus from Hanoi directly to Cat Ba town then use some local agencies to organize trips there. Bus ticket costs around 12 USD per person per way. Hotel costs around 15 USD per night included breakfast. This is the cheapest way to explore Cat Ba island however there is not really safe as many accidents were happen like falling down mountains or ocean, getting lost in the park… 
The second option is taking a tour Halong bay and Cat Ba island 3 days 2 nights, you will stay the first overnight on boat then the second night on Cat Ba island. Tour itinerary takes you to do kayak and visit cave in Halong bay, then biking in Cat Ba national park. It’s interesting and easy for guests who want to save time and enjoy the most incredible place in the north of Vietnam. Trip costs rank from 150 USD per person depends on quality and services. There is an option to staying the second night in bungalow on Monkey island where is less tourist and beautiful nature. 

Top things to see and do on Cat Ba island:
• Cat Ba national park: The more challenging hikes include a trip up to Ao Ech (Frog Lake) and over the hill to a tribal village inhabited by the Viet Hai. 
• Beaches: The first beach is called Cat Co 3, to reach this one by going along the harbor road West until ends, and then going over the headland. If you head West on the harbor road but turn up the hill on the left before the Harbour View Hotel, you can see Cat Co 1, straight ahead then see Cat Co 2 by turning left towards Cat Co Beach Resort. Both are quieter than Cat Co 3. You can also walk to Cat Co 1 from Cat Co 3, and it’s worth the walk, with fantastic views over the bay and the beaches.
• Hospital cave: The cave is located 10 kilometres north of Cat Ba town, inland, on the way to the national park. Hospital Cave was a secret, bomb-proof hospital built during the American War and was used as a safe house for VC leaders until 1975.
• Connon Fort:  Build atop a 177 metre-high hill overlooking Lan Ha Bay, the historical side of Cannon Fort may not appeal to everyone but it’s worth the trek up there for the views alone.
• Rock climbing and boat trip in Lan Ha Bay: Can be organized on Cat Ba island by many travel agencies.