Ya' haven't said what anniversary it is; 50th is much different to the 1st! If you have a nice picture of them, it could be put in a nice frame...or a few pictures could make a collage. Or even have one exploded onto canvas...
A plaque or trophy stating how long they've made it together.
A massage for two is always nice (but that's similar to a gift card).
If they have a child/children, then having the kids for a weekend would be an amazing gift!
A nice new bedding set.
You could put a basket together, containing candles, massage oil, champagne and strawberries...and/or something kinky of you and they are comfortable with that sort of thing (a karma sutra book, handcuffs, body chocolate, blindfold etc).
...just a few ideas :-) x