Maybe something like the group was sent to destroy a monster (something big like a dragon), but when they destroy the dragon they don't find is treasure trove but a clue to it's location, the clues lead them to a kingdom where a princess is in need of cure for a mysterious curse so they agree to gather the items if they find them along the way to the treasure, when the find the dragon's trove there is a book or something hinting at the location of the fountain of youth so a new adventure begins.