By Daniel Kline The cruise line’s “Perfect Day” concept has given it something Carnival and Norwegian can’t offer. Cruise line private islands generally just offer a beach, maybe a few beach activities, perhaps a kiddie play area, various bars, and a BBQ-style lunch. That’s what Carnival Cruises Lines (CCL) – Get Carnival Corporation Report and Norwegian Cruise Line (NCLH) – Get Norwegian Cruise Line Holdings Ltd. Report offer. Some cruisers prefer these beach-based islands that offer a nice day at or in the ocean and not much else. Royal Caribbean (RCL) – Get Royal Caribbean Group Report deci…