If one does things that are worth doing, then even doing them forever wouldn't be boring.
So the underlying issue seems to be that those who complain that eternity would be monotonous are currently living lives which they believe are not worth living ...
Say you liked building. So in eternity you started to build.
You could build a Cathedral for every atom to sit in, a palace for every quark.
When you had finished, with your eternity hardly yet begun, what would you do next?