I commuted once with my 50-oz Platypus pack (no bladder). There was room for my aerobics clothes (shorts and light top) ONLY - shoes and CD's live in my locker at the gym, and I wore the same bra and socks for my class that I did to ride. Also I picked up two packages of pita bread from the store. I really had to jam them in. No way would long pants fit in there.

For shoes, I used to sometimes carry those flat cloth Chinese shoes. They hardly take any space - less than flip flops even, and you don't have to take your socks off to put them on.