I have been using my trusty vineyard vines tote mostly, and here are a few things that will pretty much be guaranteed to be inside.
1. My planner, I always need to know where I'm supposed to be and what needs to be done
2. Pens (pencils, highlighters...) I've been using my bag as a book-bag too so I always have things I might need for class on hand.
3. Sunglasses, you never know when they may come in handy
4. My eos lip balm, isn't the little egg shape cute!
5. Kindle, to get some light reading done in my spare time, waiting for meetings and class
6. Wallet (id, money, charlie card...) I think this is a given.
7. Phone... another given, this mine as well be glued to my hand
8. Ipod, always nice to have something to listen too on the way to class or something (when of course I have it charged, I'm really bad about keeping my charged and updated)
9. Band aids (yes, I sometimes have hello kitty band aids) you always need one when you don't have one, paper cuts, blisters from new shoes, or if you trip and fall... safety first! :)