We loooooved Hoi An. Really want to go back. Lovely beach (not the tourist one, the locals one - as it was described to us) with great bars you can sit in and relax all day and enjoy food and drink and watch people herd goats up the beach!
We cycled up to it and stayed all day.
My DH got a suit made there, which is still going about 8 years later.
Great food. Also recommend the morning glory restaurant.
Hanoi was super polluted on certain days so take a good mask if you can. It was overwhelming.
We did a fantastic street food tour in Hanoi which was really worth doing - we would have had no idea where to go otherwise. You must have the banh mi!
Not thinking of going to Ha Long Bay? We did an amazing boat tour. We did some cookery, kayaking. Ate some amazing food. Obviously fantastic scenery! Well worth it but I wouldn't go for the cheapest option of this as have heard some horror stories.
I also quite liked Ho Chi Minh City. It was interesting, particularly the War Musuem which I wish we'd had more time at.
Amazing country! Have a really great time!