I did a master's in urban planning at Cornell a few years ago. I only looked seriously at three programs (Cornell, MIT, and USC), but I don't recall any of those programs having hard prerequ requirements. Your undergrad major would be fine for Cornell without any additional coursework unless things have changed in just the last few years.
Instead of taking additional classes, I'd advise you to get some work or volunteer experience. Business improvement districts and other small economic/community development organizations love to put volunteers to work. Just a few hours per week in something like that will be a bigger boost to your resume than any additional coursework. It could also give you something more substantive to write about in your application letter.
If you have decent quantitative skills, that will give you a big step up in the application process. Planning schools are full of people who love policy, but hate numbers. Showing that you have the skills for real quantitative analysis can set you apart. I had a relatively weak undergrad GPA, but I think that my solid math GRE score and my more quantitative undergrad coursework really helped me to get into a better school than my GPA alone would have suggested.
If you're looking for something more design oriented, landscape architecture might be another option. Also, planning schools vary in the extent to which they lean toward design vs. policy. Cornell tends to be policy oriented (though there is incredible freedom in electives that allows you to really tailor your program to whatever interests you have. I believe that Harvard is a very urban design oriented program.