I'll suggest that you write a note. Use paper.
You can add a small gift to this - something that she can keep on her desk, a souvenir perhaps, that is related to her passion/subject.
Other than the great advice above, simply in terms of writing the thank you note, you may find this link useful:
I'll mention the main points here, and you can use the link for a more elaborate guide.
a. Always write the note as soon as possible.
b. Send it through the mail.
c. Use real stationery.
i) Begin by expressing your gratitude for the gift/service.
ii) Mention specific details about how you plan to use a gift or what you enjoyed about an experience.
iii) For some recipients, add some news about your life.
iv) Close by referencing the past and alluding to the future.
v) Repeat your thanks. (in short)
I hope this is useful in penning the actual note.