We had no extra costs other than if you wanted to purchase beverages on the cruise. There was no visitation to islands. We left the boat only to kayak if desired or to partake in a short boat tour through a cave. Not sure if your trip includes something different though or is with a different company....

No age limit, as long as their parents go with it

The weather was perfect in February. I heard rain is coming around March. So, how is weather in May I don’t know. The activities were not difficult. We had 89 year old with us, she didn’t do kayaking but she went on bamboo boat caving. We didn’t go to swim. We are oldies. We are just amazed with the scenery. We love kayaking. This is the highlight. I believe, you parents can do all the activities. I saw others doing it. And, the boat was not crowded. The only issue we face was their English that hard for us to understand and we just needed to ask for second time. I hope the operator can answer your question about weather in May

We were supposed to be picked up in Hanoi but were already in Ha long and it was easy to communicate that with them and just meet them at the cruise dock. It was all quite casual so i assume it would be fine if you just asked them to not take you back on the bus and instead sorted your own taxi to Ha long. However I don’t think you’d be able to book a bus back to Hanoi through them but there’s plenty other buses you could just get on your own the next day, Hope that helps :)

The transport is fine. Big touring bus, but there are a few stairs. The transfer to the boat/cruise might be a little tricky as it’s from a tender boat (smaller) and you transfer on the water. My mother is 73 and was ok, they lay down anti slip mats and will assist if necessary. There are stairs on the cruise between floors, I don’t remember seeing an elevator. Maybe ask the cruise line directly if assistance is available. The tours off the boat has a lot of walking so perhaps there is another tour/option that is more suitable. Beautiful place to see though.

We only traveled with 3 but there was plenty of room for our luggage (2 pieces each). We got back to Hanoi around 2:30pm. It’s 2.5 hours each way with light traffic and the cruise ended around noon.