Something totally luxurious that they wouldn't dream of spending on for themselves.
How about an experience day? Being driven in a vintage car to a hotel to have a cream tea.
Or if they're a modern type a his and her spa day.
A scrapbook of their marriage (you can contact a local scrap booking group to see if anyone will do this for you though you need be quick).
Memorabilia for the year, newspaper, cd of the all the music in the charts that year.