Sending a well-prepared thank-you letter is a very good way to express your gratitude. A thank-you note can express what you have done. In addition, you can also consider the following gifts: 1. Books or magazines related to your research field. 2. A beautifully decorated notebook or laptop bag. 3. Unforgettable souvenirs, such as customized glass sculptures or personalized seals or stationery. 4. Favorite high-quality chocolate, coffee or tea. 5. Favorite music CD or movie DVD. 6. Handmade scarves, gloves or hats. No matter what gift you choose, the most important thing is to express your gratitude and respect your guidance and support.