Gold is a symbol of fire; fire is a symbol of passion. One heart bound to another for a half century deserves a tribute that reflects the glow of a lifelong affection. At this age -- and through the subsequent anniversary years -- a couple values family above all; grown children may have children of their own by now, and all will want to participate in the honoring of Grandmother and Grandfather's marriage by sharing memories.
Here is the idea for this simple golden memory party: At every table, have pretty note cards on which each guest writes his or her favorite memories of the couple and places them in a large jar designated for that purpose. During dessert, pass the jar around; one at a time, guests take out a card, read it, and then try to guess whose memory it is. This is a wonderful way to learn details of a couple's life through their friends and family. At the end of the party, place the cards in a book made of envelopes created by gluing each flap to the next envelope (use archival tape to reinforce). Tie with a gold ribbon, and present it to the golden couple.