If you can go online or look around in some antique shops, try to find some avacado green dishes. Avacado green was very big in the 70's. Also fondue, as has been mentioned, crepes, quiche.
Also try asking your older relatives what was served.
Try to dress the part too with plenty of polyester and bell bottoms. Wouldn't be too hard since that stuff is back in style.
VH1 had a show called I Love the 70's. You could check out the websight to see what was in, also.