I highly recommend doing an street-food walking tour in Hanoi, Tra Que eco-village visit in Hoi An, Vespa tour in Ho Chi Minh City and Coconut Island in Mekong Delta as they are all super interesting and amazing experiences that truly show the local Vietnam. We visited Sapa, finished the trekking tour with Lim, who is a great tribal young girl but the trek was a little hard for us (I blame it on our poor health at 70 but we insisted doing this, and no regret at all).
It's such a great feeling when you go there, blending in the maniac but fun traffic, talking to local people getting to know their lives. We got to taste a lot of local cuisines and they are so tasty, no wonder why people are obsessed with Vietnamese foods! I am so grateful to take this trip into the local life and many other Vietnamese aspects so we saw the true Vietnam.
Two days one night in Halong Bay cruise should be enough.
Get a hotel in Hoi An town instead of along the beach as most of the hotels offer shuttle or bike to get around.
Mekong Delta is a must see.
We stayed at Ancient House Resort in Hoi An and it was absolutely brilliant place, warm and attentive staff. We will come back in Hoi An, such a wonderful place to relax and enjoy the peaceful atmosphere.