We have just published our Ho Chi Minh City 3 Day Itinerary. This is what we would do with only 1 day in Ho Chi Minh City:
Morning: War Remnants Museum and Downtown (use this free walking tour to be time efficient.)
Afternoon: Chinatown (check out the Cholon "Chinatown" Free Tour in Ho Chi Minh)
Evening: Rooftop bar and Bui Vien Street.