My first recommendation is for you to work with a reputable travel agent who is a cruise specialist and can answer all you questions and help you find the best cruise for your parents.
Now, with that said, I would assume your parents are probably in their 50's based on celebrating their 30th anniversary. Personally, I think they would really love Holland America or Celebrity if they want a more sedate cruise with great food and wonderful service. If they'd like a little more activity and still good food, you might want to look at Princess.
It really does depend on their lifestyle and what they like to do. Some people prefer a more relaxing cruise, while others want alot of activities.
Again, if you work with an agent who is a cruise specialist, their assistance can be invaluable in situations just like this and the prices will be exactly the same or maybe even better than what the cruise line offers. Plus, they can assist with what to do in each port.