K.C.
How about a letter from the president? You can send a request for it and then have it framed for them.
A collection of 60 things about the day they were married ... Written or printed from the computer in a nice book with some accompanying pictures to represent the 60 things. Google could help find things about that day ... The headlines, popular songs, movies, etc.
Do you knit or crochet? A diamond motif afghan would be pretty. Or even a small quilt ...
A rose bush with diamond in the name.
Just a few things.