Cat Ba Island and National Park
Cat Ba is the largest island in the Bay and approximately half of its area is covered by a National Park, which is home to the highly endangered Cat Ba langur. This golden-headed langur is rarely seen, as fewer than 100 specimens are thought to survive in the wild, although it is the subject of a well-organised conservation programme. The Park covers both land and marine areas and has a high biodiversity, although it is at risk from too rapid an increase in tourism. Other mammals in the Park include civet cats and oriental giant squirrels.
Although there is much beauty to explore on the island, Cat Ba town itself is rather crummy: construction, massage parlours, blaring music, touts and drunk tourists.
You can visit the National Park by heading north up the main road from the town. This is the domain of the endangered Cat Ba Langur, of which there are supposed to be only around 100 left. Things on offer include:
Walk to a peak viewing tower. Very slippery and muddy if it's rained recently; in this case expect to get dirty. It's possibly better to do it in bare feet.
A frog pond (didn't visit, probably better in the off (wet) season
A short nature walk near the frog pond
More stuff which you can probably find online...