If you only have a half day off, you can take MRT to Tamsui station. Tamsui is in the north of Taipei. You can set the beautiful sunset there.
In addition, there are many famous food in Taisui, such as Arab League. It is a special food made by fried beans and sweet potato starch noodles. It is delicious with its sweet and spicy source. You can go Tamsui to find your favorite food in there and enjoy the unique sunset.