Do you know if they have a favorite charity? If so, you could just make a donation in their name to it and put a note saying that in a card.
My father was beginning to fail health-wise by my parents 50th (although most didn't know), and my mother had started having dementia problems (again, not known yet to most except for the immediate family), so even gift cards for things like a dinner out weren't particularly useful - the reality was that they didn't go out often at that point, and almost never without one of us kids involved. For the 50th, they really just wanted to celebrate their anniversary with family and friends, gifts weren't needed or expected.
One other idea, if you just have to give a "thing", would be something like a nice bottle of wine or champagne, if you know that they drink. My parents always appreciated a good bottle of wine even in the depth of health issues! LOL