The Collaborative International Dictionary of English v.0.48:

Whimsical \Whim"si*cal\, a. [From Whimsey.]
   [1913 Webster]
   1. Full of, or characterized by, whims; actuated by a whim;
      having peculiar notions; queer; strange; freakish. "A
      whimsical insult." --Macaulay.
      [1913 Webster]

            My neighbors call me whimsical.       --Addison.
      [1913 Webster]

   2. Odd or fantastic in appearance; quaintly devised;
      fantastic. "A whimsical chair." --Evelyn.
      [1913 Webster]

   Syn: Quaint; capricious; fanciful; fantastic.
        [1913 Webster]

WordNet (r) 3.0 (2006):

whimsical
    adj 1: determined by chance or impulse or whim rather than by
           necessity or reason; "a capricious refusal";
           "authoritarian rulers are frequently capricious"; "the
           victim of whimsical persecutions" [syn: capricious,
           impulsive, whimsical]

Moby Thesaurus II by Grady Ward, 1.0:

198 Moby Thesaurus words for "whimsical":
   Attic, Gothic, abnormal, absurd, adrift, afloat, agnostic,
   alternating, ambiguous, amorphous, amusing, anomalous, arbitrary,
   baroque, biting, bizarre, brain-born, brilliant, capricious,
   chancy, changeable, changeful, clever, crank, crankish, cranky,
   crotchety, desultory, deviable, deviant, deviative, dicey,
   different, divergent, dizzy, dotty, doubting, dream-built, droll,
   eccentric, equivocal, erratic, exceptional, extravagant, facetious,
   fanciful, fancy-born, fancy-built, fancy-woven, fantasied,
   fantasque, fantastic, fast and loose, fey, fickle, fitful, flaky,
   flickering, flighty, flitting, florid, fluctuant, fluctuating,
   freakish, funny, giddy, grotesque, harebrained, hesitant,
   hesitating, hilarious, humorous, humorsome, idiocratic,
   idiosyncratic, iffy, impetuous, impulsive, incalculable,
   incongruous, inconsistent, inconstant, indecisive, indemonstrable,
   infirm, irregular, irresolute, irresponsible, jesting, jocose,
   jocular, joking, joky, joshing, keen, keen-witted, kinky, kooky,
   laughable, ludicrous, maggoty, mazy, mercurial, moody, mordant,
   motiveless, nimble-witted, notional, nutty, odd, oddball,
   outlandish, peculiar, petulant, pointed, polysemous, preposterous,
   priceless, pungent, quaint, queer, quick-witted, quirky, quizzical,
   rambling, rapier-like, restless, rich, ridiculous, risible, rococo,
   roving, salt, salty, scatterbrained, scintillating, screaming,
   screwball, screwy, shapeless, sharp, shifting, shifty, shuffling,
   singular, skeptical, smart, sparkling, spasmodic, spineless,
   sprightly, strange, temperamental, touch-and-go, twisted,
   unaccountable, uncertain, unconfirmable, uncontrolled,
   unconventional, unconvinced, undependable, undisciplined,
   undivinable, unfixed, unforeseeable, unnatural, unpersuaded,
   unpredictable, unprovable, unreasonable, unreliable, unrestrained,
   unsettled, unstable, unstable as water, unstaid, unsteadfast,
   unsteady, unsure, unverifiable, vacillating, vagarious, vagrant,
   variable, vicissitudinary, vicissitudinous, volatile, wacky,
   wandering, wanton, wavering, wavery, wavy, wayward, wild,
   wishy-washy, witty