It's not necessary to buy the dinner package to get a seat - especially if you are going early in December. You may have to spend some wait time in line though. If you are going the week of Christmas, you pretty much need a package to be assured of getting a seat. The weekday shows are less crowded than those on the weekends.
My advice is to arrange to be near the theater about 1 1/2 to 2 hours ahead of show time. Check out the theater to see how much of it is reserved for package/VIP guests. Then keep an eye on the line for stand by guests (right side of the theater if you are facing the water), while you enjoy exploring the areas around the American Pavillion. When it looks like the line is getting long enough to fill the non-reserved seats, get in line. You may want to buy something to snack on while you are in line.
FWIW, the main beauty of the CP is the narration/music - there's really not all that much to actually see. You can hear the program quite well from outside the theater. Many people choose to stand just outside the theater or to sit at one of the tables outside the American Pavillion's counter service restaurant. Many parents of young children choose this last - their little ones can eat and move about a bit without disturbing those in the theater.
Enjoy your trip! The CP is a definite "don't miss" for us!