If "fun" means enjoyable:
* Dull: Lacking excitement or interest.
* Boring: Uninteresting and tedious.
* Monotonous: Lacking variety and stimulating.
* Tedious: Long and tiresome.
* Tiresome: Causing fatigue or weariness.
* Unpleasant: Causing discomfort or displeasure.
If "fun" means playful and lighthearted:
* Serious: Important, solemn, and demanding attention.
* Solemn: Formal and serious in manner.
* Grim: Dark and depressing.
* Dour: Gloomy and unfriendly.
If "fun" means exciting and stimulating:
* Boring: Uninteresting and tedious.
* Monotonous: Lacking variety and stimulating.
* Dull: Lacking excitement or interest.
* Staid: Calm, serious, and conventional.
If "fun" means enjoyable for a group:
* Lonely: Sad because you are alone.
* Isolated: Separated from others.
* Solitary: Done or enjoyed alone.
The best word to use depends on the context and the specific meaning of "fun" you want to convey.