What classes to take in undergraduate for graduate school instead of graduating early
It really depends on where you go for graduate school. Some schools may accept graduate courses from other places, some will allow you to use them to test out of prerequisites, some won't accept them at all.

That being said, taking gradate courses as a undergrad will look good on your gradate school applications. Depending upon your subject, if you can also manage to swing some undergrad research experience, your gradate school applications will be in really good shape.