“ A casual remark by one of the party about something met on a previous adventure leads to a summons to and grilling by the Mages' Guild (or equivalent), probably resulting in an expedition back to that area to recover the things for study by the Guild. The party's presence will be 'requested' for this expedition.”
“ Enraged by the violation of the Peace Pool and the Whalebone Forest, a dead leviathan has risen from the bottom of the ocean and now hounds ships around the port where the stolen jewels went to land. It capsizes ships and devours sailors whole, spears protuding from its dead and crab-eaten hide.”
“ The gold hoops and rings that sailors wear are to throw into the sea if there is a storm. These are bribes to the sea gods to let them live through the storm.”