Wow, 8 years doesn't seem like all that long to hold a renewal ceremony - maybe it feels a lot longer to them! (I've been married for 20 years and it feels like 8.)
Let's hope they're not trolling for gifts. I like B00's idea of a restaurant gift card. Or maybe try a donation in their names to a charity they support.
If they like movies, maybe their local cinema sells gift cards. When I worked in a college, one of the student employees in our office got married, and we gave him a gift card to his local video rental store, a basket of sweets and a big book of a movie critic's film reviews.