Great advice from Nancy! In addition, if your budget will allow, you might check out Seabourn, Silversea, Crystal or Radisson Seven Seas cruises, or Oceania (though it's a brand new cruise line and no reviews will exist quite yet, the ships are from the old Renaissance line and I have heard very good reports of the inaugural cruise). All but Oceania are "luxury" lines with higher price tags, and will be more likely to cater to couples, and have less of a "party" atmosphere, with an emphasis on service and pampering. Oceania is also emphasizing service and a feeling of casual luxury, but at lower prices than the others I mentioned.
A cruise is a great way to celebrate an anniversary, in my opinion. Congratulations!