The Collaborative International Dictionary of English v.0.48:

Restful \Rest"ful\ (r?st"f?l), a.
   1. Being at rest; quiet. --Shak.
      [1913 Webster]

   2. Giving rest; freeing from toil, trouble, etc.
      [1913 Webster]

            Tired with all these, for restful death I cry.
                                                  --Shak.
      [1913 Webster] -- Rest"ful*ly, adv. -- Rest"ful*ness,
      n.
      [1913 Webster]

WordNet (r) 3.0 (2006):

restful
    adj 1: affording physical or mental rest; "she spent a restful
           night at home" [syn: restful, reposeful, relaxing]
           [ant: restless, uneasy]

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "restful":
   appeasing, at peace, at rest, bloodless, calm, calming, cloistered,
   comfortable, comforting, comfy, commodious, concordant, contented,
   convenient, cool, cozy, cradling, cushioned, cushiony, cushy,
   dreamy, drowsy, dwindling, easeful, easy, ebbing, even-tenored,
   festal, friendly, gentling, halcyon, holiday, homelike, homely,
   homey, hushed, hushing, hypnotic, idyllic, impassive, isolated,
   lived-in, lulling, luxurious, moldering, mollifying, orderly,
   pacific, pacifying, pastoral, peaceable, peaceful, peacetime,
   piping, placid, quiescent, quiet, quietening, relaxed, relaxing,
   reposeful, reposing, resting, rocking, roomy, sabbatical, secluded,
   sedative, sequestered, sequestrated, serene, sheltered,
   sleep-inducing, smooth, snug, soft, somnolent, soothful, soothing,
   soporific, still, still as death, stilling, stillish, stilly,
   stoic, stolid, subsiding, tranquil, tranquilizing, unagitated,
   undisturbed, unmoved, unperturbed, unruffled, unstirring,
   untroubled, vacational, waning, warm