College courses to learn more aspects of machine learning

I recently entered a PhD program in computer engineering where I have the opportunity to take any courses I want across discipline (and I don't have to pay). While my main focus is signal processing, I like what I have learned about machine learning as an undergrad and would like to continue learning. (and hopefully apply it)

Obviously, statistics and computer science (i.e. database courses/data mining and ML) in conjunction with my signal processing courses would be desired. So, my question to you, do you have any recommendations- what should I focus on? What would you do with this opportunity?

Thanks, I know its a little open ended, but I appreciate some guidance.