Qaqortoq is a small town and easily navigable on foot. Most of the main attractions are within walking distance. Taxis are also available, but it is recommended to negotiate the price before getting into the cab. The travel cost would be around 50 kr per ride.
If you have more time, try a boat tour to the nearby ice fjords or visit the Southern Greenland National Park. If you have less time, skip the Museum of Qaqortoq or Hvalsey Church.