My current quest is to take something from the business domain I work in and implement the bizarre math stuff (which I don't get all of) in a backend in F# with a frontend in Javascript (I'm fluent in neither).

I've tried it before (with different technologies), and although it didn't make me an expert in either field, I've found it neatly ties most things together.

Oh, and it's usually fun, especially if you put somewhat bizarre obstacles in your way (remember, it's a learning experience, not production code). It could be something elaborate like http://www.xpteam.com/jeff/writings/objectcalisthenics.rtf or just a concious decision to implement a part of the design in a part of the system where it isn't most obviously at home.