While travelling around Ha Long Bay, it is possible to visit the south side of the island on a day trip from the boat, then a short(ish) bike ride. The island is beautiful. It is the largest island in the bay and is the only one with established villages on (as far as I know based on what our guides told us). Our first visit to the island was to a small remote village where we cycled around and visited the school, the nurses station, the school and a little road-side tea house.
Cat Ba Island contains a designated national park which can easily be accessed by bus or moped. The park itself is more or less central to the island and has been managed well in so far at the trails up to the view-points are pretty easy to flow and there are options if you fancy a harder steeper climb. The drive to the park entrance is simple as there are only 3 main roads on the island so getting lost is pretty difficult but looking around while driving has it's dangers but is virtually impossible to avoid as the scenery is wonderful especially once you get to the very top. The final section of the path contains some pretty ropey ladders and some sharp rocks to climb but the effort is definitely worth it.
Cat Ba Island is considered by many to be the 'Party Island' and we could imaging that being the case given the strip of coastline overlooking the main harbour with numbers bars and restaurants along the waterfront. Oh, and a massive (probably 6-8 storey high) neon sign in the middle!! Our visit was in the off season so it was very quiet but a great opportunity to see the island at our own pace and we didn't feel the need to party each night in the bars along the main drag. Everyone was amazingly friendly - as seemed to be the Vietnamese way and me got to know a few of the people working in the bars and restaurants over the 3 days we stayed there.