We have normally heard of seven wonders of the world. Halong Bay is one of the seven natural wonders of the world! Visible at a distance from the harbor itself, it is best seen by spending atleast few hours in emerald green water surrounded by limestone formation arising from nowhere. If you can then spend a night or two in night.
Halong Bay is ~2.5hr – 3hr mins drive from Hanoi. Depending on time available with you and your desire to be surrounded by water you can select one day trips starting at $35 for solo / small groups to luxury cruises of over $400 for two nights three day trips. I selected somewhere in the middle at around $120
Typical itinerary for two nights and three days trip goes like this:
Day 1: Leave from Hanoi at 8:30AM. Board the cruise ship and go into Halong Bay. Enjoy the views as you move away from the city. In second half of the day, stop to see Sung Sot caves and kayak to see some tunnels and swim in Halong Bay to the disappearing sun. First night is spent on the Boat.
Day 2: Move towards Catba islands. On the way see fishing villages and do more kayaking or swimming or simply sun bathing on the ship. On reaching the islands, go to the national park and hike your way up to the view point. Come down and be driven to check into the hotel. Enjoy the evening exploring the town and local food.
Day 3: Board the ship and see more fishing villages / kayaking / sun bath to tunnels before de-boarding in late afternoon for shuttle transfer to Hanoi. Reach Hanoi between 5PM – 6PM.
There are boats / cruise ships available from Halong Bay itself for day cruise, but given the well organized tours from Hanoi, it makes much more sense to start from here and not from Halong Bay on DIY trip. Even from Halong Bay, you will be selecting a tour by buying ticket in a 25 or 40 seater cruise ships. And they are 25 or 40 seater in theory only with overcrowding.
Since you will in sea for three days with no alternate options available, it is very critical that you select your tour operator with great care. You will not like to spoil your trip to Vietnam and three day tour to Halong Bay by saving few dollars for bad food / arrangement. Google / reference check your potential tour operator before signing up.