You could make a book with pictures throughout their relationship. You could ask them questions (withouth them knowing what you're doing) about how they met and write up stories to include. I just started making these books and they are a huge hit. Go to www.staceykerr.com and it will take you to the website-Heritage Makers. Best of all, they are so easy to make and great quality.
A plate is a great idea, too. No doubt they would love anything that has your children's hand or foot prints on them. I use Timla Healy with Painted Piggies. You can go to her website at www.paintedpiggies.com. She does a fabulous job.
Another note - if you do a picture with Natalie Ann - she is great and very affordable. When I was a kid, we surprised my parents with a picture of my brother, sister and me. My parents were so surprised and absolutely loved it.