This is my current "only the essentials," take everywhere kit packed into a Think Tank Photo Streetwalker backpack. I'd prefer a shoulder bag but backpacks lay gear out in a single level and that's what I'm used to and prefer.
I never wear any of my photo backpacks like a backpack. I just carry them by the handle and transfer some of the gear to a small shoulder bag if I need to carry equipment covered for any distance.
Here is an iPhone 6 Plus photo of the bag followed by a description of the gear it contains:
Starting at the bottom left and continuing clockwise in the main compartment we have my EOS 5D Mark III with an EF 24-70mm f/2.8L USM lens attached.
Next is an EF 28-105mm f/3.5-4.5 II USM lens with lens shade and a small size rocket blower.
Next is an EF 40mm f/2.8 STM, EF 50mm f/1.8 STM and ProOptic 20mm extension tube attached to an OP/Tech dual end cap and a Canon 270EX II Speedlite with STO-FEN Omni Bounce.
Next is a Canon Remote Switch RS-80N3 and a Canon Anti-Fog Eyepiece in its case.
Next is an EF 70-200mm f/2.8L USM lens with lens hood.
Next is a Canon 77mm 500D closeup filter, Hoya PRO1 77mm filters of the UV and CPL variety along with a Tiffen 77mm 3 stop neutral density filter.
Lastly is a spare Canon LP-E6 battery and body cap and lens end cap. Missing from this compartment is a Greenbatteries.com nylon battery wallet and 4 Eneloop AA batteries which are in the charger at the moment.
The zippered sections contain a jar opener for filter use, a swab and cloth which comes with the anti-fog eyepiece, 58mm to 77mm filter adapter, lens wipes and a lens cloth.
The bag's all weather cover is in an outside compartment. I never understood keeping the cover inside the bag so if you need it in the rain you have to open the bag. This leaves some outside pockets empty for whatever.