I've been to both HCMC and Hanoi. I actually like both of them equally. Also, both have roads that are crazy-filled with motorcycles. The first time I visited (Hanoi was first), it took me 10 minutes to cross a street.
After that, I learned the secret of crossing streets (traffic lights are ignored by drivers/riders, don't wait for traffic to clear as it will never happen, just walk across slow and steady, don't make sudden moves, and keep an eye on the bikes coming towards you and ensure they see you), and it was all smooth sailing.
I took the 2D1N Halung Bay trip. I think that's enough, as you would have gone around Halung bay and seen much of it. I spent 1 night sleeping in the ship moored in calm waters. Very clean and comfy, but the bathroom was super small. Important: ask for a room AWAY from the engine room for peace and quiet. The cruise itself was very enjoyable as long as the weather is good. Just relax on the sun-chairs on the ship's deck and enjoy the blue skies, awesome seascape and impressive rock formations. It's heart-breaking to see pollution in the water (patches of floating plastic and rubbish are fairly common) amongst such natural beauty.
From Hanoi, you can also take a day trip to Tam Coc (1 day) to see the limestone karsts(nice, though not as spectacular as Halung Bay), and also a trip to Sapa (I suggest 3D2N), which is a totally different experience and highly recommended.
Small, clean, decent hotels in Hanoi are around 30-40 USD. Obviously for this price, you are not getting the Intercontinental or Hilton. These are typically private hotels with around 10-rooms with aircon, TV, attached bath, a proper reception, bell boy, and usually includes breakfast. You will find lots of options in Agoda.com.