Boredom will be the least of your worries in fact we choose on several "port days" to stay on board, Reason being that the ship was less crowded & lots to do. Most expensive are the guided tours ashore. In most ports you are better off making your own arrangements when you go ashore. Best deals onboard are on the last full day when the shops & stalls have cut price offers. Jewellers, wine shops, cigarettes, souveniers, sportswear & art are the main shopping items on board.
One tip - Do not seek to be first to embark & fiorst to disembark unless you absolutely have to. If you leave things till later the long lines will all be gone.