Research your cruise company carefully before you make a decision - for example the Celebrity Cruise Line concentrates on fine food and gracious living, Royal Caribbean Cruise Lines provide lots of activities and recreational facilities and some of the other cruise lines have offerings which may/may not be to your parents' taste, such as "let's all get to know each other" sessions, wet t-shirt competitions etc.
www.cruisecritic.com and
www.cruisemates.com are good sites for passenger reviews.
Personally my choice will always be Royal Caribbean, having experienced 6 different cruises with them. For your parents I would recommend a week based in Vancouver or touring around Canada and a week on an Alaskan cruise for an unforgettable experience. The Fairmont Hotel chain in Canada is excellent.
The Balkan Peninsula may be attractive from them also, as they will get to visit Norway, Sweden, Russia, Estonia, Finland and Denmark. They might like to spend a few days in London before the cruise and take the ship from Southampton.
The Med cruises are very popular now, so a few days in Madrid and/or around the Barcelona area before or after a cruise would be lovely also. I have found the Melia Hotel chain to be very good, or for something a little different they can stay in the paradores,
www.paradores-spain.com. RCCL are offering Med cruises leaving from Southampton.
I normally book cruise/hotels/flights directly.