If you have 7 hearts with the Harvest Goddess, she will give you a present if you speak to her on your birthday. There are three different presents available, depending on the year. She will give you either a Pineapple (Year 1), Firefly Flower (Year 2) or a Strawberry (Year 3). These gifts will continue in a set rotation every three years.
If she gives you Pineapple, you can gift it back to her for a major boost of affection (or save it for her birthday in your refrigerator for even more heart points).