Despite having a fairl hefty Pentax K20, typically with a weighty Tamrom 18-200 zoom and polarizing filter, I actually bought a Think Tank Mirroless Mover 30i before the trip to Canada last year. Despite being designed for a mirrorless system but it holds my camera and my sigma 70-300 zoom plus a few small accessories, battery extra cards etc. It also has a slot at the back probably designed to take an ipad mini but it is just fine for an A5 sketch block or A5 Visual diary, so I also slipped in an old tin of Neocolour II water-soluble crayons. I like using them, they have good intense colours once activated by water and I’ve had these for several years but haven’t been able to fine a local supplier so I tend to use then on and off. The discovery of water brushes means I don’t have to carry water but I do also pack in a couple of brushes, pencils and/or pens. All this takes up very little room and almost no extra weight (depends on thickness and paper weight of the sketch book mainly) so I can leave it in my camera bag at all times.