If you want the freedom to go around without to have to take a taxi every-time, renting a car is the best option. Also is cost efficient if you are a family.
Lots of things to explore around Khao Lak, but all attraction are a bit far away, so you need a bike or a car or take a taxi.
You can also rent a car from Phuket airport on your arrival and bring it back on your departure. All the nice hotels in Khao Lak have parking area.
The best option for car rental is:
Rent a car in Khao Lak or Phuket or anywhere in the world.