You could do rows of seating like an airplane.
Then serve their lunch on a tray. Make it so that it's easy. Not a tray they
have to hold. They have TV trays you can buy (look @ thrift stores first).
You could even put cardboard along the rows of seats w/circle cut outs
for windows.
Hand out wing pins like they do on airplanes.
Someone can dress as the captain & someone as the stewardess.
You can even have a drink cart (use whatever you have).
Get a remote control airplane you can fly in the backyard (buy a cheap one).
Give out little packs of peanuts (make sure no nut allergies first).
Play pin the propeller on the airplane.
Give out tiny decks of playing cards.
Decorate the table w/little airplanes.
Get little foam airplanes you put together & have a flying (throwing) conest outside.
Make announcements into a cb looking speaker "This is your captain. Fasten your seatbelts. We are headed for blue skies & birthday fun". (etc.
Have fun.