As the biggest mall in Toronto, the Eaton's Centre has just about everything. Encompassing 3 floors and over 3 blocks of street space it is often quite a bit of a challenge to find your way around. But if you're looking for a day of shopping there is no better place. There are subway stations on either side of the mall (north and south), so getting there is easy. If possible, don't bother parking as the rates are excessive and driving anywhere on Yonge Street can be a bit of a challenge for those who don't know the one way streets.
Mall is generally busy, especially on weekends and the weeks before Christmas, so if you don't like crowds give it a miss during those periods. Also make sure you pick up a floor map if you're looking for a specific store. If it's a nice day it's also a good idea to walk out to Yonge and go to the many stores along there.