I think spending a big anniversary on a cruise ship is a great way to celebrate!! Just makes it all that much more special.
We did our 40th anniversary last year on the Paul Gauguin in Tahiti. It was awesome and such a memorable way to celebrate together in such a magnificent setting. Very romantic!
Make sure you plan to spend one full day doing some really special for yourselves; start off with breakfast on the balcony, then treat yourselves to a nice steam bath and massage in the spa, followed by a romantic dinner in one of the specialty restaurants with a nice bottle of wine. (And don't forget to have flowers delivered to your cabin - she'll love it!)
And depending on where you are and which cruise line you choose, perhaps you can do a renewing of your vows. We did that on the Paul Gauguin with Mt. Otemanu, Bora Bora in the background. It was magical!
Travel Agent/Cruise Specialist w/12 yrs exp and 47 Cruises on 11 cruise lines! Favorites: Paul Gauguin - Tahiti: Uniworld River Cruises - Europe; Celebrity Solstice-class ships; Holland America - 12-nights Baltics & Russia; RCCL - 14-nights Greek Isles, Turkey, & Croatia; Holland America - 14-day Alaskan cruisetour; 10-night Canada/New England cruise; 21 days Hawaii w/7-night NCL cruise; Oceania - 25 days in Asia; more than 3 months touring Europe by train. And many all-inclusive resorts!