Perhaps you've heard of Winter's Gem, and of the tragic fate of the Lady Karina. And perhaps not. It is, after all, a tale unspeakably old. Yet the Earth does not forget, and those of us who guard it do not wither...
Nine thousand years she has slumbered, while Winter diminished. So obsessed was He in her care, that He allowed Summer to run unchecked. His power dwindled, and Summer grew. There is a great unbalance in the world, young ones. One that I fear will break the back of the Earth itself, if not addressed. Winter must return. It is the Way.
A real-world location fit for filling out your adventure; where norse icelandic fishermen suffered unthinkable conditions throughout frigid winters to harvest the fruits of the ocean.
In the summer there is a small stone jail for those who are caught breaking the rules of the Sorcery Springs Geyser Basin, but in winter they are imprisoned within the warm waters themselves. No bolts or bars are needed, the cold takes the place of prison walls.
The Yugzhee, or "hedgehog-people", guard a great, secret treasure inside their giant burrowed lair. The mysterious Wall of Keys, a 12'x10', foot-thick, cold-iron, "wall", upon which, on iron hooks, hang 100 ornate iron keys of all shapes and sizes. Each key opens some heretofore un-openable barrier, door, or gate, in the particular game world of choice.
The PCs have come upon a great boon, except for the fact that the keys cease to function properly if separated from the iron wall for more than a few minutes. Higher level characters will be able to figure out how to take the wall "with them" via magic. Lower level characters will have to get creative.