Talking to traders or, if possible, sit on a trading desk. Any macro course is going to be very useful. The relationship between interest rates, currencies, inflation etc. is very topical. The world economy is so much more connected now than it was when I was an undergrad, so any international economics course would be great. Not the super advanced theoretical stuff but basic calculus or statistics is a big help.
Any good trader has to have a good handle on math.
I was fortunate enough to play intercollegiate athletics and that was a tremedous experience and helpful later on. Knowing how to win and lose the proper way,interaction with your teamates,pulling for each other. I realize that not everyone will participate on that level but try intramurals or a non-sport group where you have to work with and relate to different groups of people. That will be helpful after your college days. I wish I took more classes that were totally unrelated to my major.