If you're interesting in the continuous side of mathematics, MATH3405 is an essential course. It describes how calculus can be done on surfaces without reference to anything but the surface.
MATH3405 uses the language of MATH2301 and MATH2400. However, the assessment is computational (think MATH2001), and things like differential forms and derivations are not discussed. Huy follows the required text Do Carmo extremely closely, so your learning and grade is ultimately in your own hands. His approach to the course is to have an easy final exam, but to massage the grade distribution using challenging assignments.
Personally, I was not too interested in pursuing further studies in the field of geometry, but I still found the course invaluable because it teaches you how linear algebra and analysis interact: how eigenvectors are used for optimization, a rigorous understanding of the Jacobian and Hessian matrices, the meaning and context for a conformal mapping from MATH3401.
It also advances your understanding of linear algebra and analysis. For example you develop your understanding of bilinear forms, orthogonal matrices, self adjoint matrices, and how the idea of a change of basis from linear algebra is a specific form of the more general idea of a change of coordinates which does not rely on ideas from linear algebra. You also develop your understanding of the MATH3401 Riemann sphere/stereographic projection, of the multivariable Taylor series, and learn to appreciate how other shapes e.g. a sphere/circle can be used to approximate shapes, just like a line is used to approximate shapes in the case of the derivative.