You pick from the regular menu, or the full buffet is included if you're at a buffet.
The only restricting issues I can think of are 1) lobster entrees are excluded where they are served and 2) Via Napoli allows an individual pizza as an entree, but package users can't pool their orders to get a larger pizza to share.
I am also not sure why you would want to pay full price for a dining package (not even on DDP) and then only order an appetizer and a drink. Those packages are expensive, they're about five times what an appetizer and a drink would normally cost. I am guessing you would be allowed to ask for less than the package allows, but there won't be any reduction in the price.