Here's a cute idea. I saw this at a wedding, but a 50th sounds like a great fit, too.
Get a nice looking blank notebook, or even just some index cards. Ask every guest at the dinner to write down a few sentences regarding a memory they have of the couple or the individuals, for the couple to read the next day.
It's touching and surprising what people remember about how that couple helped their friends in the past, or good times had years ago. (Or in this case, potentially decades ago)