Final answer:
Fanny Mendelssohn Hensel was known for her compositions and performances, but not for conducting. There is no evidence to suggest she was involved in conducting, making it the activity that does not describe her musical activities.
Step-by-step explanation:
The activity that does not describe Fanny Mendelssohn Hensel's musical activities is conducting. Fanny Mendelssohn was a prominent composer, pianist, and organist during the early 19th century. While known for her compositions and performance skills, there is no evidence to suggest that she was involved in conducting. As for music theory, while she was certainly knowledgeable about it, being a composer and performer, her direct contributions to the field of music theory as a discipline are not as well recognized as her work in composition and performance.