I don't think boredom played a part in it (though I can't tell if you're being facetious). The Elvish mind would be somewhat different from the human mind, but if Elves got bored they would probably do something.

When Tolkien says the Eldar attempted nothing new in the Third Age, he's speaking of the creation of new realms and artifacts. The Elves did, in fact, create new things: songs and stories.

But the Eldar of the Third Age were considerably fewer than the Eldar of previous ages. I don't think people appreciate just how numerous Gil-galad's people must have been throughout the Second Age. They were the dominant civilization in Middle-earth. They were most likely the largest, most populous race in the Northwest after the War of the Elves and Sauron.

But after the War of the Elves and Sauron the greatest of the Noldor in "arts and crafts" (what we might call the crafting of magical things) either departed or were killed. Some truly great and powerful Elves remained (Cirdan, Gil-galad, Glorfindel, Galadriel, Elrond, Celeborn, et. al.), but these were not the crafters who made great artifacts. The Feanorians were the Elves who devised incredible objects and worked wonders which moved the world. They were gone after the War of the Elves and Sauron.

And after the War of the Last Alliance, there were probably only a handful of truly wise and powerful Elves who were even capable of doing great things. Galadriel, Glorfindel, and maybe Gildor Inglorion would have been among the last great Elves in Middle-earth. The pool of talent, as it were, shrank considerably, and the Elves probably came to understand before the end of the Second Age that their attempts to meddle with the nature of the world were far too destructive to pursue anything else on that scale again. At least, outside of Aman.
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