For one thing, your food for the shift ought to be in there. Water, soda, juice, etc. I don't carry a backpack but I sure pack the heck out of my lunchbox. If you pack decent food and enough of it, plus fluids, your brain will work better.
Also, don't forget your :
smallish bag containing all the crap that goes in the pockets, like scissors, hemostats, carpuject, fingernail clippers, calipers, etc.
stash of pens and highlighters
a book in case patients don't come to the ER at all (can't say the Q word! and the book might jinx it completely anyhow)
your own bottle of whatever you take for a headache
and all your techy stuff: phone, palm, etc.
reference books if you don't have a PDA
Try and keep the weight under 20 pounds, food included. Your Mileage May Vary.