You CAN just pay the price for the cruise and be happy.
Additional things that add to the cost would be alcoholic drinks, fancy coffee, popcorn at the movies, WiFi, gels to, Palo the fancy restaurant, the spa, and excursions/tours. You can pre pay suggested gratuities.
Then there is the cost of getting there and possibly
DCL transportation to the port.
But I have actually not bought anything extra and had a great time. Meals are included, free soft serve ice cream, and all the entertainment (and great people watching) is included. You can walk around any port stops on your own, etc.
There ARE a couple books about cruising with Disney on
Amazon if you prefer books over Google.