Rock climbing trips can be arranged in Cat Ba, led by experienced climbers.
Talk to Asia Outdoors in The Good Bar building which can organise trips for all levels.
Asia Outdoors can also organise kayaking, as can other agents in town. Expect to pay around $15 for a few hours, or combine kayaking with a boat trip for a full day experience.
Small boats cruise the harbour looking for fares, so if you prefer to be closer to the water but don't want to do the hard paddling, you can give it a whirl.
Motorbikes are available to rent all over the place for 80,000 VND to 120,000 VND – both automatic and semi-automatic – and it's a good way to explore the island. The road conditions aren't great – although there was a lot of construction work going on when we visited in August 2014 – but roads are quiet and the scenery is amazing enough you'll want to keep stopping to take photos.
Bicycles are also available to rent but bear in mind Cat Ba is quite hilly, particularly in the centre, so you'll need a certain degree of fitness. Or rent a tandem for 50,000 VND per hour for a gentle ride along the harbour road. They actually have four seats (two for children) so you can make it a family outing.