There are so many caves in Halong Bay, I just mention the most popular ones. In Route one just for 1-day boat trip cruising, you may visit Thien Cung Grotto (heavenly palace). This is huge and enhanced by chains of lights inside with different shapes of animals.
Thien Cung Cave, Halong Bay
They are formed due to millions of years of rock erosion by water, creating system of stalagmites and stalactites on both ceiling and grounds. Then you can explore Ba Hang by sampan boat rowed by locals. Its passage is only 2 meters high, that means you need to bow your head. Further than this, you may approach Sung Sot cave (Surprising Cave), however, you can only get there by overnight cruises or book with We Go Halong (the only day cruises with itinerary to visit this cave and Titov island). By the side of Sung Sot cave, you can rent a kayak to Bo Nau cave which is located just opposite to Sung Sot cave. Please put on shoes and spray insect repellents because snakes and mosquitos are quite popular there.
Amazing Cave, Halong Bay
Up to the North, Hang Trong (Drum cave) is accessible for its fame of being the home for events with dining and wine party for big groups who wish to have unique dining inside the cave. Near Cat Ba island, there are Hang Sang, Hang Toi (bright and dark cave). These are for those who love to explore its mystery by adventure kayaks.