It depends a little on how long I'm going to be out photographing, but I generally want to carry as little as possible assuming that I'm going to be out and carrying everything for a few hours at least. Travel light.
My current bag is the excellent ThinkTank Retrospective 5 which gives me plenty of space for all I need, plus a tablet if I'm teaching a workshop. And this is what I pack it with:
- One camera, one lens. Currently the Fujifilm X-T10 with the 35mm f/2 lens.
- Phone (although it's usually in a pocket instead). Currently the LG Nexus 5X.
- Spare batteries (at least one, usually two or three).
- Power brick and cable for the phone.
- Spare SD cards. I use 32GB high-speed cards by Lexar.
- Lens cleaning cloth.
- Business cards.
That's it, plus some loose dollars in my pocket. Simple and light. I might take water, depending on the weather, location and whether I feel like carrying the extra weight. I've probably regretted the extra weight more often than not though.