3 Weeks Vietnam and Cambodia - Itinerary advice

Hi, I'm flying into Hanoi November 1st, and I have about 3 weeks to spend between Vietnam. I have some stops in mind but nothing is set in stone. My biggest interest is nature, but I would also like to see some culture and history. I'm not so interested in big cities, but I am intrigued by Hanoi. I would like to do one beach, but not more. I'm not too worried about weather, as I was just in Laos during the rainy season and it didn't affect my travel much. If anything I preferred it because everything was so green and there were hardly any tourists.

Here's what's on my radar already
Hanoi: 3-4 Nights
Cao Bang: 2-3 Nights
Halong Bay: 1 Night
Hoi An: 3 Nights
Dalat: 3 Nights
Ankor: 3 Nights

Please let me know of any other must sees, or anything I may want to skip. Thanks.