In April I hosted a group of 53 on
Royal Caribbean's Voyager of the Seas. In a word....Amazing! No complaints from any of the group. This ship (and her sister ships) are the largest ships in the world next to the Queen Mary 2 which is just slightly larger. It features in addition to the usual pools, clubs, kids facilities, spas etc., an ice skating rink (where you can see an awesome ice show), rock climbing wall, in line skating, mini golf the most gorgeous dining room on land or sea, and a four story royal promenade which features a shopping mall, pub, ice cream facility, and cafe. The entertainment included everything from the normal vegas/broadway type shows to comedians, game shows, production shows (the Platter's performed when we were on board) and the aforementioned ice show. If they haven't cruised before they really should learn all they can about cruising in advance of the cruise. Activities ran the gamut from art auctions to bingo to cooking demonstrations, the usual contests, exercise and dance classes, and much more.
In short, I would definitely recommend Royal Caribbean's Voyager or her sister ships Explorer, Navigator, Mariner or Adventurer. Beware however that these ships will spoil them. They will be hooked on cruising forever!