Suppose there was a physical button next to the screen to enter the order system, and the system could recognize the users’s seat number.

—  The essential color is dark grey, which works like the background of vending machine. The secondary color is brown, which is the metaphor of dining-table.
—  For the side menu, I choose orange to representative the color of drink, and green color to representative salad and its dressing. Sandwich and snacks are made of grain, so I use grey color. And dessert is always colorful, so it’s purple.)

So here is my design and the interaction:

 

Details on the airplane seatback ordering system:

A PASSENGER SHOULD BE ABLE TO ORDER

• Soda (4 types), juice (2 types), coffee, tea water
• (and ice/no ice, can/no can, coffee w or w/o milk (whole or skim), sugar,)
• Sandwich (3 types)
• Salad (2 types)
• Dressing (4 types), mayo/mustard
• Snacks (4 types of chips) or nuts or fruit
• Dessert: cookies, yogurt (3 flavors)

And also:
Identify quantity

Place their order: 3 payment options or cash

Cancel their order

Change order

Or signal not to be bothered.