You haven't indicated a budget for this but it should be something that they both enjoy, obviously, and here's two ideas.....
First up some kind of trip... weekend in Paris... two nights in a nice country hotel somewhere... an activity weekend somewhere...
Secondly there's the sort of "here's a thing" gift but you've already said they have everything and I assume that you refer to the sort of household and personal stuff there. How about a piece of art? What about something for their garden?
Just a couple of thoughts... I'm sure some more imaginative ideas will pop up shortly.
Rob