Vietnam, Thailand, Cambodia

Hi!

I'm looking for advice for a couple (F34, F41)

My plan is to travel as follows:

12th of December early morning-15th of Dec - Hanoi,

15th of Dec late evening - 20th of Dec - Chiang Mai (tickets already booked)

No tickets for rest:

20th of December - 25th of Dec - Koh Samui (travel though Surat Thani and take ferry)

25th of December - 29th of Dec - Bangkok (again ferry to Surat Thani and flight to Bangkok)

29th of December - 1st of January - Siem Reap (night bus to the next stop)

1st of January - 4th of Jan - Phnom Penh,

4th of January - ??? Ho Chi Minh City.

I need to travel back home on the 11th of January.

I really wanted to see Hoi An, is it possible? I know it may be rainy, so not really sure if it's worth it. Should I stay somewhere longer and finish in Ho Chi Minh City?

I would have to travel back home from Ho Chi Minh City.

Any other suggestions are very welcome.

We are more onto quiet holidays, don't really like partying or crowded cities.

Would love to stay somewhere for a couple of relaxing days hence Koh Samui.

We are both foodies, love nature, hiking and history.

Mid budget.

Thanks in advance!