If you do harvest your pineapple a tad prematurely then it can be left on the kitchen counter for a few days. A pineapple will not continue to ripen once it has been picked and won't get sweeter in taste, however it will get softer in texture over the course of a day or two at room temperature. An underripe pineapple can be cooked or grilled to help bring out the sugars, however very unripe pineapples can be potentially toxic to eat.