- Measure theory / lebesque/daniell integration / stochastic calculus -- super useful but very beautiful. I have a background in mathematical finance.
- Combinatorial topology -- Simplicial complexes, polytopes. A more finite/computational flavor of algebraic topology.
- Dynamical systems: Highly interdisciplinary. Brings together physics, fractals, calculus, and computer simulations.
- Multilinear Algebra -- tensors, grassman algebras.
- History of Mathematics -- love reading about the development of mathematics throughout the centuries.