I'd go with Hindi!

I used to be fluent in Bahasa (spoken and read) and be able to read Italian, French and German (making conversation was another matter).

All were pretty easy, expect for the French!

BUT if you are going to actually use it, it's in the top 3 spoken languages in the world. And becuase it's got so many sub-languages, like Creole, you can be understood pretty much anywhere.

I want to go with Cantonese next.

TAFE classes are pretty awesome apparently...it's how my Mum made her French fluent again. However, I personally like to go private one-on-one tutoring. That way you're not forced to go with the same rate as the rest of the class.