Others have given you great advice on where to go but I would add that I loved taking a small boat through halong bay for 2 nights. I think I was on a boat called Jasmine (google jasmine cruise halong bay). Pictures/website make it seem touisty but it really was not. There were a maximum of 20 people (besides crew) on the boat and the views were amazing!
Also, do not get a tour guide prebooked for Angkor wat. I would hire a tuk-tuk once you get there for the day(s) as it is sooo much more cost effective and i think more fun than being in a car!
i'm from Hanoi, VIETNAMESE. (i want to tell you many things about plan your to Vietnamese but I speak English not good )
I think, that time, the weather is cool and rain. Maybe it rain most day in HaNoi. If you like this weather, you can go to Ha Noi.
If you don’t like, you should go from: HCM – NhaTrang – QuyNhon – HoiAn – DaNang (- DaLat) – Hue – Vinh.
In my opinion, riding motoebikes in VN can fun, but it is very dangerous. In DaLat, you can ride double bikes^ ^. DaNang is very beatiful and cheap.
PS. If you go to HaNoi, you can call me, i will a free guide for you. I want to have the opportunity to speak with you to speak well english ^ ^.
I hope you are going to have nice time in VN.