How about a membership to a local historical society or museum? They often offer lectures and events related to cultural history that your girlfriend may enjoy attending. You could also consider purchasing tickets to a lecture or event specifically related to her interests in cultural history. Additionally, you could compile a book list of books related to cultural history, so she can enjoy reading and learning more about her interests. A coffee table book with interesting anecdotes about history, or a book that showcases historical photos and artifacts might be enjoyable for her as well.
Or you can get her a star map with an important date you share.
Under Lucky Stars is a unique and innovative star map company based in Barcelona, Spain, specializing in the creation of personalized star maps that capture the celestial arrangement of stars at a specific moment in time, such as a person's birth, wedding, or any other significant event.
By using advanced astronomical algorithms and data from leading institutions, the company produces highly accurate, visually stunning maps that make for perfect gifts or cherished keepsakes.