The Collaborative International Dictionary of English v.0.48:

Light-hearted \Light"-heart`ed\ (l[imac]t"h[aum]rt`[e^]d), a.
   Free from grief or anxiety; gay; cheerful; merry. --
   Light"-heart`ed*ly, adv. -- Light"-heart`ed*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

light-hearted
    adj 1: carefree and happy and lighthearted; "was loved for her
           blithe spirit"; "a merry blithesome nature"; "her
           lighthearted nature"; "trilling songs with a lightsome
           heart" [syn: blithe, blithesome, lighthearted,
           lightsome, light-hearted]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "lighthearted":
   airy, blithe, blithesome, breezy, buoyant, carefree, corky,
   debonair, effervescent, expansive, festive, free, free and easy,
   gay, glad, gleeful, happy, high-spirited, insouciant, jaunty,
   jocund, jolly, jovial, joyful, joyous, light, lightsome, lively,
   minded, mirthful, perky, resilient, spirited, sprightly, vivacious,
   volatile