ON A BEACH.
Varkala & Goa (India), Palawan (Philippines), Bay Islands (Honduras) or Ko Taoet al. (Small, touristy, Thai island)
On A Boat.
Mekong, Niger or Yangtze Kiang slow boat. Also the Nile Feluccas (probably best to avoid public vessels if you want to chill)
By The Lake.
Lake Malawi (Nakata or Monkey bay), Lake Atitlán or El Estor (Guatemala) and Lake Titicaca (Bolivia/Peru).
Others.
Kotor, Montenegro. A huge inlet with a quiet town.
Yangshuo, China. A peaceful and mystical island.