If you want to visit the island of Cat Ba, I recommend renting a motorbike. If you do not feel comfortable riding your own bike, you can take one of the bike taxis. Although it is possible to rent a bike in the street -- many local guys rent their own motorbikes --, you'd better ask your hotel.
The island has some small fishing villages of interest. With the motorbikes you can also get to some of the beaches near Cat Ba Town. Some of them are really nice.