Unconventional subjects that candidates can choose in the UPSC Mains include Anthropology, Public Administration, and Geography. These subjects may not be traditionally associated with the civil services but offer unique perspectives and methodologies that can aid candidates in their preparation. Others might choose Literature subjects or Philosophy for a more analytical approach, allowing them to capitalize on their interests and strengths. Choosing an unconventional subject can lead to a personalized approach tailored to the candidate's unique skill set.