Some professionals carry around a briefcase, I choose a backpack. As a video producer, I sometimes have to work strange hours, travel last-minute, and always have to be prepared for just about anything and I try to make my daily bag reflect that.
1 - My bag...a standard, neutral-colored Jansport backpack - This thing has LOTS of room! Not only can it fit everything else in the photo, there's still plenty of space for my shaving kit, an extra pair of shoes, and a couple-day's worth of clothes. I like the blandness of the color/style because it makes it pretty unnoticeable and therefor less likely to get snatched.
2 - clipboard/folder/notebook - The clipboard is obviously useful, as is the notebook (Moleskin, college-ruled lines, with a pocket for loose notes). The folder contains (usually) drafts of various scripts I'm working on as well as outlines, poems, stories, doodles, etc.
3 - sketch journal - Doodling helps me think, occupies my hands and eyes while I mull other things...and I love my little Moleskin sketch journal.
4 - personal journal - This is my third one of these. It's less of a "Dear Diary" and more of a "here's this crazy thought or here's this crazy thing I saw" kind of journal.
5 - 7" Kindle Fire HD - A basic tablet. Mostly I use it for watching videos I've uploaded to it or Netflix (assuming I have access to wifi). Also, for reading books and magazines.
6 - USB flash drives (various capacities) - Dang useful when moving files and media from one computer to another when network sharing or FTP isn't available.
7 - bootable Linux USB flash drive - This is a recent addition. Basically allows me to use any computer with my own documents and files without needing any special passwords or access permissions. It's loaded with Ubuntu and includes basic software (Firefox, Open Office, etc.) as well as a few useful tools to help people trouble-shooting their systems.
8 - 1 TB UBS hard drive - On this I keep my collection of stock video, compositing effects, production music, sound effects (foley), stock photos, clip art, etc...as well as an archive of my own completed work.
9 - 9 ft. grounded extension cord - Uses: obvious.
10 - 300 amp DC-to-AC power invertor - Also has obvious uses...but also includes a USB port.
11 - 20 oz. stainless steel water bottle - Easy to clean, easy to chill, hydration enabler.
12 - flashlight - It's really bright (3x LED) and nearly indestructible.
13 - work gloves - Video production work sometimes requires some manual labor and protecting one's hands is important.
14 - tools - I carry a trio of inexpensive multitools for adjusting tripods and other equipment...a knife/box cutter combo (Kobolt), a small generic leatherman, and a knock-off Swiss Army knife. I like them cheap so if one breaks or is lost (or is stolen) it's easy to replace.
15 - compact reusable grocery bag - This thing has so many potential uses. More often than not I end up using it to haul dirty clothes back home.
16 - carabiner - It's one of those things that's useful to have around.
17 - camping/hiking survival tool - I keep this clipped to one of the zippers on my bag. It includes: a magnifying glass, compass, whistle, and thermometer.
18 - earbuds (with mircrophone) - This is my extra pair.
19 - Samsung Galaxy Note 3 (Sprint 4G with hands-free earbuds) - My phone and go-to device for everything from phone calls, e-mail, and texting to games to scheduling to reading to listening to music, audiobooks, and podcasts. I love the S-Pen stylus, the huge HD touch screen, the storage capacity, and the million ways the experience of using it can be customized. It's main camera is 13 mp and can shoot 1080p HD video. I use Evernote and Google Drive as my primary note-taking, planning, and writing apps. I can also use it as a wifi hotspot. The earbuds are my primary headphones.
20 - Kindle USB rapid charger - I'm serious it's THE fastest and best wall charger I've ever owned.
21 - USB cables - A 2.0 and a 3.0 cable...used mostly to charge my Note 3 and my Kindle.22 - cleaning supplies - I carry a lintless cloth and disposable alcohol wipes...for cleaning my glasses, but also my electronics.
23 - lotion (Neosporin alcohol-free) & pills (aspirin & ibuprofen) - Because you have to keep your skin moisturized and your aches & pains at bay.
24 - pens - All Sharpie...extra fine tip pens, fine tip markers, standard-sized markers, and an erasable liquid-pencil.
Some time soon, I'll show my camera bag and all of its contents. Unlike this bag, it's only usually with me, not always with me.