Are you looking for specifically healable stuff, or just non-lethal?

For the stuff that an actual human could recover from, there's:
Beating/whipping/caning,
Dripping wax (not a huge fan of this one, personally),
Rope-binding (cutting off circulation to a specific part of the body, such as breasts or cock and balls)
Minor flame play

There's also the breaking of bones, which can heal, but wouldn't be done irl because of how long the recovery time is

For stuff that won't fully heal, but can be survived with little impact on someone's life, there is:
Knife play/scarification,
Branding,
Piercing (as in, inserting semi-permanent rings/bars of metal into the skin as decoration)

As far as reasons to do it, there's the idea that some people have of "I'll try anything once!"

A lot of the stuff I mentioned can also be part of master/slave roleplay pretty easily

There's also the potential of people competing to withstand the most pain before chickening out