My favorite item is my White Coat clipboard Nursing Edition. It folds in half, so it is HIPAA compliant. My current clinical instructor does not allow us to carry anything in our hands while we are on the unit, so I use this because it fits nicely into my scrubs pocket. The information on the outside is very useful, but I have to be careful to show it to all instructors for lab checkoffs and simulations, because some don't allow this cheat sheet.
Amazon.com : WhiteCoat Clipboard- Wine - Nursing Edition : Office Products
It is also useful to have some kind of pouch for your pens. Think pocket protector. I carry a minimum of 1 black pen, 1 4-color pen, 1 sharpie, and my penlight. Our uniform is white top and bottom, so it keeps me from ruining the white scrubs with ink stains. At the end of the day, I also drop the whole pouch into a basket with the other things I keep in my pockets during clinicals, so it is ready to go again the next clinical day. Oh and alcohol wipes or hand sanitizer is great for getting pen stains out of white scrubs.
I have a Bat Clip to keep my stethoscope on my waist. Right before I started nursing school, a PACU nurse told me about a patient who was confused and combative when coming out of anesthesia. This patient grabed her stethoscope which was hanging around her neck and tried to strangle her with it. This nurse friend bought me a bat clip when I got into nursing school so that I won't ever be in the habit of wearing my stethoscope on my neck. I love it, and it holds my stethoscope very securely.
Amazon.com: The BATCLIP (black) - Premium leather handmade clip-on stethoscope hip holder; no more neck carrying, loss, or misplacement. Proudly carry your high-end stethoscope with taste and style.: Health & Personal Care
I wear knee-high compression socks every day at clinicals and it makes a significant difference. Be sure to check your school's policy on color restrictions, if any.
During Fundamentals (or whatever the first semester of nursing classes is called at your school), you may want to get an inexpensive pulse oximeter, blood pressure cuff (at least manual, possibly also automatic), and a cheap temporal thermometer. The first semester at all the schools where I live are in nursing homes, and these items are often hard to find. It will be an immense time saver to have your own tools for vitals in your backpack. I also bought a very cheap glucometer to practice at home, but I do not recommend bringing this to clinicals since it may be against facility policy.
This is my dorky suggestion, but hear me out. There have been problems with theft at every single one of the clinical sites I have attended so far, including nursing homes, long term care, rehab, acute care, and critical care. People know which bags belong to students and will go through them if given the opportunity. I don't carry anything of value in my backpack. I don't want to carry my wallet, phone, and car keys in my scrubs pockets where they could fall out or just take up space. Instead, I use a runner's belt under my scrubs to hold only these items. Everything is there against my body and secure.
Amazon.com : SPIbelt: The Original No-Bounce Running Belt for Runners, Athletes and Adventurers (Black with Titanium Zipper) : Sports & Outdoors
I have a pocket sized medication book that I sometimes carry. I used it much more in Fundamentals because our facility did not have drug books at the med carts. Hospitals will often have access to Micromedex, which you can use to look up drugs quickly, but sometimes computers are scarse and you need to know about an unfamilar drug right away. This is the smallest one I found:
218 Lippincott Pocket Drug Guide for Nurses: Amy M. Karch MSN RN: 9781496371935: Amazon.com: Books
Last but certainly not least, make an emergencies kit for your backpack. This is all stuff you'll want with you, but you will certainly not use every day. But when you need it, you'll be glad you've got it. My kit includes: Tide pen (food stains only), Shout wipes (great for non-food, non-pen stains), hair band and hairpins, pads and tampons, fingernail clippers and file, cough drops, ibuprofen, Tums, Benadryl, Dayquil, and Lidocaine patches. NONE of the medications are to be used on patients! I've only ever had to use the ibprofen for headaches and the Lidocaine patch for a pulled muscle. At my school, clinical days are a pain to make up a clinical day, so I do all I can to avoid that possibility.