I'm bored at my job. I haven't learned a thing in the year that I've been there and I'm rebuffed whenever I request to be able work on a task that I would find more interesting (after having my daily tasks finished or well-in-hand, of course), or suggest a small project that I could take on that I believe would contribute positively.
I've repeatedly expressed my frustration, especially since I've been stuck with my least favorite set of responsibilities out of the rotation we work through for far longer than the normal rotation. I've been told that I'm doing such a good job that I am extremely valuable at that station.
Long story short, I'm outta there. There are enough other reasons for me to want to leave that I'm not considering requesting additional compensation for my boredom/stated value to the company. I also know that this has been an issue in the past (I've become friendly with an old employee who quit for exactly this reason) and is quite unlikely to change (based on conversations with my senior supervisor).
I anticipate an exit interview, or at least some kind of grilling from the boss, at which time I would like to be able to explain, clearly and firmly, that I'm leaving because I'm bored out of my skull, don't see any prospects for professional advancement, and wish that I could have done a lot more for the company. I feel like I've expressed this pretty explicitly already, but I suspect that it hasn't come through as clearly as I think.
I'm looking for suggestions as to how to get these sentiments across diplomatically. I'm also willing to accept "don't bother, just tell them you're moving on" as an answer, which is actually what I'm leaning towards.