There should be an option to get a sandwich, fries (or even onion rings or other side items), and a fountain drink for one price instead of paying for each thing individually. It would sort of be like getting a combo meal at a fast food restaurant.
Have a meal deal selection at the kiosk.
Step 1, pick a main dish
Step 2, pick a side
Step 3, include a fountain drink
One price for all three, with a slight discount.