Crystal Cruises is offering a kids-sail-free promotion on voyages next summer in Northern Europe and the Mediterranean. On four June and July voyages, cruisers age 17 and under can travel for free when sharing a third berth in a stateroom with two adults.  This terrific special is good on three 10-day Baltic sailings on Crystal Serenity departing June 25, July 20 and July 30, from Stockholm, Sweden, to Tallinn, Estonia; St. Petersburg, Russia (two overnights); Helsinki, Finland; Berlin/Warnemünde, Germany; and Copenhagen, Denmark.  It’s also good on the seven-day Mediterranean sailing departing July 6 on Crystal Symphony, which visits Istanbul and Kusadasi, Turkey; Mykonos, Greece; and Sicily, Sorrento, and Rome/Civitavecchia, Italy.  The Crystal Family Memories program also offers additional free berths and perks for groups of six or more.