Here's the deal with the dining plan.
The plan is a package add-on. It is not sold separately except to DVC members. You must purchase a room reservation in the form of a package to get
DDP. (The only exception to this is DVC members booking through Member Services, and possibly cast members on occasion. They do not have to purchase a package to get a dining plan - DVC does not even offer package reservations.)
A room-only reservation is not a package, and you must purchase a package to get DDP. If you are an annual passholder or eligible for the military room discount, you may purchase a package with dining but without tickets (this is considered a ticketless package - it is not a room-only reservation). If you are not a passholder or military, you must purchase tickets as part of your package.
You can purchase as little as a one-day ticket as part of the package, but everyone on the reservation age 3 or over must purchase the same ticket.
The length of the ticket you buy or the amount of time you spend in the parks is totally irrelevant to the dining plan. The plan is based solely on how many nights you stay at your resort. If you stay 5 nights you will receive credit allotments based on 5 nights, even if you buy only a 1 day ticket. You can't buy a dining plan to cover only part of your stay, unless you split your stay into separate reservations, and if you don't fit the exceptions you would need to buy tickets on each separate reservation to be eligible for a dining plan on all of them.