Areas with a lot of muscle or fat layers to cushion the blows are safest for spanking or other corporal punishments with paddles and such. Anywhere bone or connective tissue is close to the skin is not a safe place for spanking. The buttocks and the backs of the thighs are among the safest.
The soles of the feet are another area that can take a fair amount of punishment with relative safety; classically this was called bastinado. This was usually done with a cane or rod or belt rather than bare handed.
The upper back is a time honored location for whipping or flogging, but care must be taken to avoid the spine itself as there is little more than skin to protect the vertebrae and the kidney area and lower back should be considered out of bounds.
The genitals of men or women can be gently spanked but full force could result in damaging delicate tissues and no one wants that.
Some women very much enjoy having their breasts slapped or spanked, but care should be taken around the nipples and deep bruising can cause lasting damage to the mammary glands.
Any joints like the backs of the knees, ankles, elbows etc. should be avoided. The neck and head should also be off limits. Face slapping looks great in the movies, but can actually be dangerous if not done by someone experienced.
Non-standard areas that could be safely spanked would be the upper arms, front or back or the calves. If the sub or bottom tenses stomach muscles, the tummy could be spanked also.
Disclaimer: I am not a medical professional. Any new area you are trying for spanking or similar start light and build up slowly. Keep communication open and have fun.