What counts as a synonym for interested, interesting, etc. depends on your intent. Based on how you've used interesting in the posts on this page, I am going to hazard a guess and say that one of your uses of interesting is to indicate that you are somewhat uncertain about something, e.g., maybe it is unclear what the other person has said, or uncertain about how to reply. The point here is not to psycho-analyze, but rather to suggest that, instead of trying to find a synonym for interesting, perhaps you might consider simply using it less often, or even choose to simply not say anything if it's something that's unimportant to you.
In any case, even if you do find suitable synonyms, eventually you'll get tired of those, too, and sooner-or-later, we will run out of English words to suggest, which will lead to all of us spending countless hours making up new words for you, which will lead to significantly lower productivity world-wide, which will lead to an eventual global financial meltdown, global chaos, and the end of civilization. Which would be interesting, I guess. :-)