WordNet (r) 3.0 (2006):

upbeat
    adj 1: pleasantly (even unrealistically) optimistic [syn:
           cheerful, pollyannaish, upbeat]
    n 1: a contented state of being happy and healthy and
         prosperous; "the town was finally on the upbeat after our
         recent troubles" [syn: wellbeing, well-being,
         welfare, upbeat, eudaemonia, eudaimonia] [ant:
         ill-being]
    2: an unaccented beat (especially the last beat of a measure)
       [syn: upbeat, offbeat]

Moby Thesaurus II by Grady Ward, 1.0:

80 Moby Thesaurus words for "upbeat":
   Great Leap Forward, Leibnizian, advance, advancement, amelioration,
   amendment, arsis, ascent, bar beat, beat, bettering, betterment,
   boost, bout, bright, buoyant, cheerful, chiliastic, circle,
   circuit, couleur de rose, course, cycle, diastole, downbeat,
   encouraging, enhancement, enrichment, eugenics, euthenics,
   favorable, fond, furtherance, headway, heartening, improvement,
   lift, light-hearted, melioration, mend, mending, millenarian,
   millennialistic, offbeat, optimistic, perfectibilitarian,
   perfectionist, pickup, positive, preferment, progress, progression,
   promotion, pulsation, pulse, recovery, restoration, revival,
   revolution, rise, rose-colored, roseate, rosy, rotation, round,
   sanguine, series, spell, sunny, systole, thesis, throb, turn,
   uplift, upping, upswing, uptrend, upward mobility, utopian,
   wheel