I'd take a 4-5 night cruise that hits coco cay - then you get one stop that will have plenty of fun for the kids without really having an excursion expense for those who don't want one. Others can pay for the water park if that's their thing, but no one has to in order to have a good time.
I think you can get one of those 4-5 night cruises for under $450, depending on the date and port.
That would be my choice.