I strongly advise you to ask people to send you their memories (and pictures, etc.), ahead of time, not write them out at the party. Email or snail mail or whatever. You'll get much better contributions; many won't do them at all if they have to do them at the party, or just scribble out "best wishes" or something. No one wants to spend the time it would take to write something good while they are at a party. Then you can assemble them into an album and give it to your parents for a gift at the party.
We did this for our parents' fortieth anniversary many years ago. They really, really loved it, and as a bonus, the guests did, also. Several of them thanked us for the opportunity to look through old photos and recall happy memories. Some made artwork; one even sent me his high school yearbook! I love looking at that album whenever I visit. My husband did the same thing for me for my 50th birthday, and I adore it, too. Both times it was done as a surprise.
I am sure that they will love this absolutely unique gift as much as my family has.