Challenge does not always translate straight to deadly threat. Sometimes, you want to get to the player - not the player character. This requires NOT killing the PC.
Yes - you read that right. The goal is not to kill the adventurers. The goal is to get the player involved in the adventure. If it makes you feel any better, consider the fact that a dead PC is far more difficult to torment than a live PC. Often, it is the very real possibility of death or defeat that brings the greatest thrill. Here are some examples of certain-death encounters for low-to-mid-level characters that are possibly less than they seem at a glance.
· No...tell me that isn't a - beholder...with a metal ring attached by piercing to its back and trailing a length of broken chain behind. The creature has a few missing eyestalks that were bitten off and is very much the worse for wear. Not only do we have a truly terrible monster that is more on par with the lower-level PCs, but we also have a plot hook that can inspire even more dread. Who could chain up a beholder and maim the thing in the process? And, more importantly, where the heck are they now?!
· Ceiling block trap turns curiously non-fatal when it is found that the stone has somehow become brittle and crumbling with age. Sure, whoever gets hit with the block still takes a little damage - but not enough to kill them...probably. This helps keep the players nice and paranoid, without the harsh lesson of an outright death. Also - what the heck happened to the stone of the block?
· Plunging to your death? Don't worry! This giant spider's web will break your fall.
· Iron golem?! Here?! We're all gonna die! Maybe. But, consider how slow the thing is moving - and how the joints shriek with protest at every motion. What's that coming up behind the golem? Not another golem?! No. It is a rust monster, come to finish the job it started. Make sure the rusting golem gets a swipe or two at the PCs, but give the rust monster a chance to "save" them...before it goes after the heroes and their precious metal items.