[dctaflin]

Two things: German and Differential Geometry.

German is just because it's fun and it's my best second language by far, so I'd like to get fully fluent.

Differential Geometry is a diversion from working through Gravitation by Misner, Thorne, et al. I found I needed more math. Now it has kind of become an end in itself, however. It's the hardest thing I've studied since college (which was a long time ago).