Visit Cat Ba Island, Haiphong, buy a tour to Halong Bay
The atmosphere is good throughout the trip, more than 90% of Vietnamese people are friendly (the other 10% are those who seek benefits)
Get off the plane at Haiphong, take a ferry, cross to Cat Ba Island National Park, relax for 2-3 nights
Buy a tour to Halong Bay for 700-800 baht with a delicious lunch on the boat
Very good value, very beautiful