Get some noshing snacks that are still around, but don't have a real high profile so that many people will say either "What are those?" or "Oh, they still have those?", such as Munchos and Bugles, and Screaming Yellow Zonkers and Fiddle Faddle.
Also, since everyone was young during the 70s, I'd say very brightly decorated cookies and cupcakes.
Lines of powdered sugar on the coffee table would be funny. But if someone calls the cops, you'd probably have to go downtown until the situation got cleared up.