The Collaborative International Dictionary of English v.0.48:

Activity \Ac*tiv"i*ty\, n.; pl. Activities. [Cf. F.
   activit['e], LL. activitas.]
   The state or quality of being active; nimbleness; agility;
   vigorous action or operation; energy; active force; as, an
   increasing variety of human activities. "The activity of
   toil." --Palfrey.
   [1913 Webster]

   Syn: Liveliness; briskness; quickness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

activity
    n 1: any specific behavior; "they avoided all recreational
         activity" [ant: inactivity]
    2: the state of being active; "his sphere of activity"; "he is
       out of action" [syn: action, activity, activeness]
       [ant: inaction, inactiveness, inactivity]
    3: an organic process that takes place in the body; "respiratory
       activity" [syn: bodily process, body process, bodily
       function, activity]
    4: (chemistry) the capacity of a substance to take part in a
       chemical reaction; "catalytic activity"
    5: a process existing in or produced by nature (rather than by
       the intent of human beings); "the action of natural forces";
       "volcanic activity" [syn: natural process, natural
       action, action, activity]
    6: the trait of being active; moving or acting rapidly and
       energetically; "the level of activity declines with age"
       [syn: activeness, activity] [ant: inactiveness,
       inactivity, inertia]

Moby Thesaurus II by Grady Ward, 1.0:

172 Moby Thesaurus words for "activity":
   act, acting, action, actions, activism, activities, acts,
   actuation, address, affair, affairs, affectation, air, animation,
   ardor, artificial radioactivity, bag, bearing, behavior,
   behavior pattern, behavioral norm, behavioral science, brio,
   briskness, business, bustle, campaign, carriage, cause, commerce,
   commitment, comportment, concern, concernment, conduct,
   contamination, crusade, culture pattern, curiage, custom,
   decontamination, demeanor, deportment, doing, doings, drive,
   dynamics, elan, employ, employment, endeavor, energy, enterprise,
   enthusiasm, exercise, exercising, exertion, faith, fallout,
   folkway, function, functioning, gestures, glow, going, goings-on,
   great cause, guise, gusto, half-life, impetuosity, impetus,
   interest, issue, job, joie de vivre, kinematics, kinesipathy,
   kinesis, kinesitherapy, kinetics, labor, life, lifework,
   liveliness, lookout, lustiness, maintien, manner, manners,
   mass movement, matter, method, methodology, methods, mettle, mien,
   mobilization, modus vivendi, motion, motions, motivation, move,
   movement, movements, moves, moving, natural radioactivity,
   nuclear radiation, observable behavior, occupation, operation,
   operations, pattern, perkiness, pertness, play, poise, port, pose,
   posture, practice, praxis, presence, principle, procedure,
   proceeding, project, pursuit, radiant energy, radiation,
   radioactive radiation, radioactivity, radiocarbon dating,
   radiolucency, radiopacity, radiosensibility, radiosensitivity,
   reason for being, restlessness, robustness, running,
   saturation point, service, social science, specific activity,
   spirit, spiritedness, stir, stirring, style, swing, tactics, thing,
   tone, undertaking, unrest, velocity, venture, vigor, vim, vivacity,
   vocation, warmth, way, way of life, ways, work, working, workings,
   zest, zestfulness