We came to Ha Long to see the famous 3,000, or maybe even 6,000 islands. For this, you need to buy a tour ticket. Touts are selling them everywhere, boats leave periodically, there are hundreds of them. So look around, haggle, and ye shall find. The pleasure costs 20-50 bucks for a 2-4 hour trip. We paid something like 25 per person for 3 hours. Make sure to ask if the tour includes a visit to the cave (it almost always does). The cave isn't included in the price, but you can live with that, since the ticket costs somewhere around 2 bucks.
Ha Long also has a supermarket, so you don't have to overpay hustlers for soda. They have a fridge in there, canned coffee, and an endless amount of all sorts of grub like ice cream, dried fruits, plain sweet potato chips, nuts, and other Asian joys of life.