What about a really beautiful photo book of them through the years? You could say you are working on a family tree or something and borrow the pictures for a few days. If you could go from dating to now that would be really really nice.
I'm not sure that you have to give them a gift at all. I feel like the big anniversaries (for the kids to celebrate with their parents) are the 25th and 50th. I personally feel like wedding anniversaries are for the couple to celebrate together.
If you really do want to get them something, a card, flowers, or dinner would be nice. If you're looking for something bigger, a weekend away might be nice.
What about a really beautiful photo book of them through the years? You could say you are working on a family tree or something and borrow the pictures for a few days. If you could go from dating to now that would be really really nice.