A lava lamp would be great for a teen or someone into the less conventional. An electric blanket, or fireplace items will keep Kun warm at heart. Chen is wood, so anything made of wood would be liked by Chen. Since water produces wood, water gifts are also a good choice. Steer clear from metal gifts, as metal is destructive to Chen.
Water is the next best choice, as water produces wood.
Stay away from metal gifts, as metal is destructive to Sun. Earth produces metal, so earth type gifts will do good, as well. Stay away from fire gifts, as fire destroys metal. Pottery items would be appreciated, also. Earth creates metal, so earth gifts will also be good.