I have a job with more lax clothing requirements than most but it seems like you guys commute with a lot more stuff than I do.
Shoes: I leave a pair of Puma Speedcats at work. When I need to bring them back and forth for some reason they are nice and compact.
Clothes: I work in shorts and a T-shirt most days. These plus underwear and socks are half my messenger bag.
Entertainment: I generally leave something at work to read on lunch breaks. When I'm at school my commute is short enough I can ride to class in my school clothes and leave the bag totally free for other stuff.
Food: I think it is generally accepted that a granola bar, sandwich, banana and other snack item are not too large.
Rain gear: This consists of shoe covers and nothing else. I am at work for 9 hours at a time usually. If I get wet on the way in, I hang my stuff up behind my work station for it to dry. If it doesn't rain, I drape it all over my top tube.