Cruise Lines typically do a nice job of advertising their services.  But here's an idea to consider.  For the larger cruise liners, each cruise they take contains several thousand people (housed in one location).  And, though their activities on ship may be different, most vacationers will go to the same restaurants, the same shops, the same shows, bars, clubs, walk through the same main hallways or lobbies, etc., at least a few times on a typical cruise ship.  This gives the ships unbelievable access (for several days) in advertising not just what's offered on their current cruise, but other cruises their company offers worldwide.  How about having a museum like showroom (in a highly traveled area of the ship) that showcases these other cruise options; using everything from photos, local artifacts, info on lodging, food, activities on and off the ship, detailed history of the areas traveled to, previous customers responses and testimonials, etc. etc.  If placed in the right location (and displayed well), this is an easy and effective way to entice those on ship to become future, and hopefully regular, "cruisers" with your cruise line.