Keep them cool in winter and re-pot every year. Sweet Potatoes make a pretty vine that looks great on a kitchen windowsill. As with the avocado, find a jar or glass and insert toothpicks to hold the sweet potato in place, the pointed end of the sweet potato down. Make sure that the end stays covered with water. You will see results fairly soon, and within a few weeks the vine should begin to fill out.
Pineapples are bromeliads, some varieties of which can be quite expensive.
Leave the top two inches of pineapple flesh attached, and let it sit out and dry for a few days before planting. Cover it to the bottom of the leaves with a sandy, well drained mix of soil. Keep the soil moist and place the pot in a sunny spot, preferably with high humidity. After a few weeks, roots will develop and the top will begin to grow. It will eventually produce a large plant, and it is possible to get the plant to bear fruit.