FWIW, I think your primary "problem" is here. You have a certain image in your mind of what a "non-challenging encounter" is. I think (and others here have posted this too) that your perception is off.
It's entirely possible to have an encounter that is both fun and challenging to your players, without even causing 1 hp of damage. Not even 1.
It might be instructive if you (the OP) posted what you think a "non-challenging encounter" would be. I think you've called them "filler encounters" as well? Post what you think one of these would look like, as it would look like in your own campaign, with your players.
I see plenty of adventures where you first have fight A, then fight B, and only then fight C against the boss baddies (or whatever).
So the evolution of my game is this:
The first time the players have fight A, it was exciting. Soon, however, they realize fight A can't really hurt them, only shave off a surge or two.
Also, by the time we come to fight C, it's time to say good night. While we did fight C at a later date, it would really be better if the adventure could come to its climax during the same night where we started.
So gradually, I'm upping the difficulty of fight A, perhaps by including some of the monsters from fight B. Thinking that not only do I make things more exciting, I also save some time to enable fight C.
Very soon, however, the players stock up on extra healing surge triggers.
This turns fight A back into a game where the PCs are invincible. After all, they don't need enough triggers to use up the surges of every character in the party - only the one wailed on by those monsters smart enough to focus their fire.
Unless the monsters can kill a character outright in a single turn (which I don't find fun, and am not aiming for), the party can always bring him back up again.
But in this case, while there is enough time to have fight C, now the characters are low on surges. So they choose to rest. Partially because they can, partially because the difficulty of fight A scared them (into thinking if A was hard, C must be even harder).
Yes, I can do the "princess needs saving tonight" spiel. And sometimes it works, especially if I can give them an overview of the opposition already at the start.
Managing your resources, after all, isn't fun unless you have some idea of what your up against. Otherwise this "management" simply becomes "trying to not use anything".
What I need is for fight A to have a possibility of being outright dangerous. Not always, of course.
What this thread have made me realize is I need the characters to have access to less hit points. This allows excitement and drama without making fights go on and on.
Certainly not a return to the 4 hp Wizard. But more along the lines of the hp totals you're actually given by the book.
But you were asking me for an example, because...?