Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
activity [ækˈtɪvɪtɪ] n (being active) акти́вность f (action) де́ятельность f (pastime, pursuit) заня́тие
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.