Answer:
   
   Well, first you'll need to know that if they keep Kosher, they cannot
   use any utensils or plates at your house, because those are
   non-kosher. Secondly, you need to know that with respect to prepared
   products, there are little marks that indicate the products Kashrut
   status. You can get a listing of all the symbols from
   [5]http://www.kashrusmagazine.com/, but the best known and most widely
   used on is a U in a circle.
   
   Given this: You're not going to want to serve anything that requires
   heating or other preparation, as that will require using your utensils
   (an exception might be heating water in a paper cup in the microwave,
   but ask first).
   
   You can serve food in packages that they see you open, such as tea
   bags (Red Rose, Tetleys, Lipton), cookies (Stella D'oro). You can also
   serve fresh fruit, which comes in its own package.
   
   Also, let them see you open the package of paper plates, etc.
   
   Of course, the best answer is to talk to them first, and ask what is
   acceptable to them. They'll appreciate your thoughtfullness.