The best way to truly see Cat Ba Island is on the back of a motorbike. If you know how to ride one, lucky you! Otherwise find a companion or hire a driver on basically any street corner. The island is way too big and hilly to explore on foot or bicycle and there is so much to see.
Three temples dedicated to each of the fabled women that Cat Ba Island is named after are up in the hills with stunning views. There is a lookout point at Fort Canon with lots of secret bunkers from the war. A butterfly farm, a zoo, and lots of local culture can also be observed by meandering through the hillside.
My favorite place I went to on the island is the Hospital Cave near the Cat ba National Park. If you go just before sunset the light is absolutely beautiful.