Cost and convenience may play a lot into your travel decision as well. St. Lucia and Granada are a bit farther away and more difficult to get into than Montego Bay in Jamaica. With 6 days inclusive of travel, it'll likely take you most of a day to get there and most of a day to get back. You could also consider one of the resort areas of Puerto Rico or an all inclusive resort in Punta Cana in the Dominican Republic where you can leave Chicago in the morning and be on the beach by the afternoon.