Ivy League is basically a better dressed, more mature version of preppy. Khaki or grey slavks are good, with a navy blue blazer featuring monogramed gold buttons. Alternatively you could wear seersucker, like a Southern prep

First off, you want your hair to be a little longer and brushed to the side. However, be sure to keep that messy and youthful appearance. With clothing, I prefer khakis and a nice polo or button down collared shirt. Also, wearing a button down collared shirt with a polo underneath is a good casual look, that I prefer when going out with friends. Also, the traditional medium gray dress slacks and navy blazer is the extremely "Ivy League." Also, a nice striped tie to go with that is a great combination.