WordNet (r) 3.0 (2006):

perk up
    v 1: gain or regain energy; "I picked up after a nap" [syn:
         perk up, perk, percolate, pick up, gain vigor]
    2: cause to be alert and energetic; "Coffee and tea stimulate
       me"; "This herbal infusion doesn't stimulate" [syn:
       stimulate, arouse, brace, energize, energise, perk
       up] [ant: calm, de-energise, de-energize, sedate,
       tranquilize, tranquillise, tranquillize]

Moby Thesaurus II by Grady Ward, 1.0:

127 Moby Thesaurus words for "perk up":
   advance, ameliorate, amend, animate, arouse, boost, brace,
   brace up, brighten, brighten up, brisk, brisken, buck up, buoy up,
   cast up, cheer, cheer up, chirk up, come along, come on,
   convalesce, develop, dynamize, electrify, elevate, energize,
   enliven, erect, escalate, exhilarate, fire, fortify, fresh up,
   freshen, freshen up, gain, gain ground, gain strength, galvanize,
   get ahead, get along, get better, go ahead, go forward, graduate,
   grow better, hearten, heave, heft, heighten, heist, hike, hoick,
   hoist, hold up, improve, inflame, inspirit, invigorate, jazz up,
   jerk up, kindle, knock up, levitate, lift, lift up, light up,
   liven, lob, loft, look up, make headway, make progress,
   make strides, meliorate, mend, pep up, pick up, progress, quicken,
   raise, raise up, rally, reanimate, rear, rear up, recreate,
   recruit, recuperate, refresh, refreshen, regale, reinvigorate,
   renew, revitalize, revive, revivify, rise, rouse, set up, shape up,
   show improvement, sky, skyrocket, sleep it off, smarten up,
   snap up, stick up, stimulate, take heart, take off, throw up,
   turn the corner, up, upbuoy, upcast, upheave, uphoist, uphold,
   uplift, upraise, uprear, upthrow, vitalize, vivify, warm, zip up