The Collaborative International Dictionary of English v.0.48:

Playful \Play"ful\, a.
   Sportive; gamboling; frolicsome; indulging a sportive fancy;
   humorous; merry; as, a playful child; a playful writer. --
   Play"ful*ly, adv. -- Play"ful*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

playful
    adj 1: full of fun and high spirits; "playful children just let
           loose from school" [ant: serious, sober, unplayful]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "playful":
   animated, antic, arch, blithe, capersome, cheerful, coltish,
   dashing, devilish, elfish, elvish, exuberant, facetious, foolish,
   frisky, frolicsome, full of beans, full of mischief, fun-loving,
   gamesome, gay, gleeful, hearty, high-spirited, hilarious, humorous,
   impish, jesting, jocund, joking, jolly, jovial, kittenish, knavish,
   larking, larkish, lighthearted, lively, merry, mirthful,
   mischief-loving, mischievous, pixie, pixieish, pixilated, prankish,
   pranksome, pranky, puckish, roguish, rollicking, rollicksome,
   rompish, scampish, scapegrace, skittish, spirited, sportful,
   sportive, sprightly, teasing, trickish, tricksy, tricky, vital,
   vivacious, waggish, whimsical, wicked, zestful, zippy