What is the best kind of Backpack for commuting?

I will only have a 4-mile commute each way to work. I can leave clothes at work if I want, but I would like to be able to pack clothes in it just in case I change my mind about what I want to wear (woman's prerogative). I need to carry my cell phone, money, credit card, license, my lock, etc. At this point, I don't plan on taking my laptop back and forth to work. However, I may need to transport the laptop later on.

I have looked at a lot of different types of backpacks and just get more confused as to what would be the best. I'd like to keep the backpack as light as possible. Right now, I don't want to use panniers or baskets because I like having my bike "free" of stuff and ready to go when I just want to go for a short ride. I had panniers on my last bike and they were a real pain to take on and off.

Thanks for the advice in advance!!