Well, at first sight I would have said that "social" was the right choice, as confirmed by the Oxford Collocations Dictionary:
However, I've just checked on the Internet that "sociable activities" is in fact used quite a lot to refer to "socialization activities," that is, activities intended to make introverts more sociable. It is indeed strange that the adjective that describes the outcome of a process (making people sociable) is used to define the process itself.
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