I go to Ho Chi Minh and the mekong delta about 4/5 times a year for work. great place, superb food, nice people. I did once Vietnam in 3 weeks also a tourist (from north to south).
In HCMC all you need to see is in any decent Guide book. Agree with who ever recommended the
War Remnants Museum, not the most fun place but...
Else head out to Can To in the Mekong delta or Vung Tau in the south for a decent beach (not great, decent). Quite a lot of expats there due to a recent surge in the container port business (ie construction of new ports)
Never had (touch wood) any issue with pickpokets or safety.