Sa Dec is a town on the Mekong delta which is well known because it was where the writer Marguerite Duras lived during part of her childhood.
My suggestion
Try walking along the banks of the Mekong, or in the little lanes of the old town where you can meet the locals and, who knows, maybe be invited to visit one of the numerous gardens, hot houses or colourful cactus plantations.
Dien Bien Phu is the capital of the province with the same name. It was the location of a battle which took place in Tonkin from 20 November 1953 to 7 May 1954 against the French Union forces. It is considered to be the place of victory for national liberation.
Blue Hmong, Red Dao, Black Lolo... The ethnic minority groups living in the misty mountains of Northern Vietnam form a true patchwork quilt of different cultures. Going to meet them is guaranteed to be an intense trip that would only equal coming into contact with other ancestral peoples.