My players are fairly creative, and one thing I like to do is come up with a puzzle or obstacle without providing the tools needed to overcome it. It's a special joy to see them comb through their inventory of rituals and items and improvise some way to cross a gorge without a bridge or gain access to the now-waterless towers in the merman prison.
A couple times, though, they didn't rise to the challenge the way I thought they would. I confronted them with an owlbear-- a beast that was obviously too tough for them to take on-- figuring that they'd find some way to distract it or sneak by it. But all of them seemed quite ready to just up and fight the thing, even Irving, who sometimes seems hellbent on talking his way out of physical conflict.
I had them come face-to-face with a ravenous plant with vine tentacles. They just backed up and set it on fire until it burned into ashes.
Recently, I blocked their path with a gelatinous cube. They just backed up until it got stuck in a door and started hammering it with magic missiles and arrows and the like. I had to fudge a bit-- something I don't like doing-- to get them to put on their puzzle-solving hats, and felt lousy about it afterwards.
What's become apparent to me is that while my method of puzzle construction-- here's the challenge, now think of something-- does work most of the time, it does so only when the puzzle doesn't involve a living (and thus killable, combatable) creature.