mohan

I would do the cooking class in Hoi An. HoiAn is small and managable. Secret Garden Restaurant/cooking school offers classes twice a day. Morning Glory is another possibility. Those restaurants are popular, book in advance for classes and tables now. Across the street from Morning glory is Cargo, good place for coffee break and sweets. I love the view from the balcony overlooking the river.
I had better-than-Paris French baguette and chocolate croissant in Vietnam, most french influenced dessert are good so don't limit yourself to Vietnam's food only.
If you are traveling between christmas and New Year, I would book hotel and transportation now if you have not already. I usually like to book everything myself but you are dealing with high season so going thru a local agency makes sense here.
I didn't take the train so cannot comment on the trains.
I flew Vietnam Airline from HCMC to HoiAn and Hui to Hanoi. With Vietnam Airline it's OK to change dates within a year, no penalty.
Book everything right now. Hope this helps.

sf7307

Re "no penalty" on VN airlines - just to clarify, it's like Southwest Airlines -- there is no additional fee for changing your flight, but you might have to pay a higher fare. We changed two of our flights, and paid a small additional fare increase on one (Danang to HCMC) but were not charged on the other (HCMC to Siem Reap).