Hi Jill,
It sounds as if you are planning to step back in time and enjoy a bit of nostalgia at
Beaches & Cream Soda Shop at Disney's Yacht and Beach Club. This is one of my personal favorite places to dine at Walt Disney World so you have chosen wisely! Unfortunately, they do not take reservations, but depending on the time of day you plan on dining there, you may have little or no wait before you are seated. I have dined there many times in the early or late afternoon (after typical lunch times and before the dinner rush) and have been seated right away or within about 5-10 minutes! So keep that in mind.
I am also afraid to tell you that the Kitchen Sink is not included in the Disney Dining Plan. I know this because I have asked before. However, there are plenty of other very tasty treats that are included as your dessert. If you feel so inclined to ask your server, by all means try, but I fear that you will receive the same answer we did.
The last part of your question got cut off, but I am going to guess that you were asking what the sandwiches come with and the answer is either fries or fresh fruit! They do serve onion rings, but they are a la carte.
I hope this helps you plan your "blast to the past" at Beaches & Cream. Enjoy!
~Allison L