Yes, ACTIVITY is a Scrabble word!

Letter values in Scrabble US: A 1, C 3, T 1, I 1, V 4, I 1, T 1, Y 4.

Is it a valid word?

ACTIVITY is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

any specific behavior
the state of being active
an organic process that takes place in the body
(chemistry) the capacity of a substance to take part in a chemical reaction
a process existing in or produced by nature (rather than by the intent of human beings)
the trait of being active; moving or acting rapidly and energetically

Also: bodily process, body process, bodily function, natural process, natural action.