In a cave, in an incredibly cold pool of water, is a large round white stone (about 3 or 4 feet in diameter). It feels to all the world like marble, and radiates magic.
It's actually a white dragon egg. It stays in stasis, just hours from hatching, until it's heated up... to just about room temperature. Then it hatches. If your players are like mine, they'll take a big white magic rock without thinking twice; it should then hatch at exactly the worst time. My players made it all the way back to their ship, and put it in the hold, before it hatched. Great fun.