I second Sandy's recommendation of children's books for great plots. When I used to substitute teach, I'd pour over the kid's readers for stories. I found a great story about an illusory tower, which I threw at my PC's as a random encounter.
They found a myterious stone tower in the middle of a desert. Naturally they climbed up for the view. The circular stairs inside seemed to go on for ever. At the top, something unseen tried to push them off. They fled down the staris, but the way down was even longer. The door by which they'd entered was gone. Instead, the stairs stopped at an underground cavern. They had several mysterious encounters inthe caverns - fighting a troll on a swaying wooden bridge, eating a mysterious feast, seeing fantastic monsters trapped in ice, fighting a giant scorpion, having to swim out through an underground spring. I think some of them drowned, and other survived, but collapsed exhausted outside.
When they woke up, they were all lying in the desert, next to a small muddy pool. There was no tower. But in the sand closeby were a lot of their own tracks forming a deep circular rut. More investigation revealed an old troll skull, a tiny dead scorpion, and some stale old (recently gnawed) bones left by heyenas.
The only real damage they suffered was having to spend a day searching for their horses, which they'd tied to a ring on the tower before they went in. Nothing those PC's encountered ever made them angrier! The humiliation at having been fooled by something mysterious and unknown drove them crazy. They searched for the answer to the mystery of that tower for a long time.
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