I had an idea for a multi-part multi-island riddle (kind of like the Merrick riddle from hungering deep) which are dropped from skeletons (either ambient or OoS).
Killing a skeleton has a small chance go drop a key along with a riddle. You need the key to open the chest at the end and to find the chest you need to solve the riddle.
To make it worthwhile, maybe the loot at the end could be slightly less than a fort since it might be easier but should take a similar amount of time to solve.
