This could range from
understanding why some theorems are true (like the Fundamental Theorem of Algebra),
applications of topics you learnt in high school (like what is the relevance of conic sections apart from memorization),
advanced topics that you are interested in (Maclaurin's series, Non-Euclidean Geometry, Field Theory)
Interesting strange theorems (Banach-Tarski paradox, Hairy Ball Theorem, Jordan Curve Theorem)
Any suggestions of topics will be taken into consideration.