Fun

Antonyms of fun:
ennui, fatigue, labor, lassitude, toil, weariness, work, disagreeable, displeasing, distasteful, uncongenial, unlovely, unpleasant, unpleasing, unwelcome, boring, drab, dreary, dull, flat, heavy, humdrum, jading, leaden, monotonous, pedestrian, pleasureless, ponderous, stodgy, stuffy, tedious, tiresome, tiring, uninteresting, wearisome, weary, wearying.
Examples of usage:
  1. I didn't think it much fun then, and I don't now.
    "Imaginary Interviews", W. D. Howells.
  2. Wouldn't it be fun if we had a little store like that?
    "Bunny Brown and His Sister Sue Keeping Store", Laura Lee Hope.
  3. Let's have some good fun up here.
    "Marjorie's Vacation", Carolyn Wells.