Descriptive:
* The silence hung heavy in the air, punctuated only by the ticking of the clock.
* Time crawled by at a snail's pace, each minute stretching into an eternity.
* The monotonous drone of the engine became a soundtrack to my growing restlessness.
Humorous:
* My brain is so bored, it's starting to invent its own games.
* I'm so bored, I could watch paint dry… and actually enjoy it.
* This is so boring, even the flies are asleep.
Frustrated:
* I can't take this boredom anymore! I need something to do.
* My mind is screaming for stimulation, but there's nothing to be found.
* This is torture. Pure, unadulterated boredom.
Poetic:
* The stillness whispered of a thousand forgotten dreams.
* Boredom, a canvas of emptiness waiting to be filled with life.
* Time seemed to stand still, trapped in a cage of monotony.
The best sentence for you will depend on the context and the feeling you want to evoke.