The first trick is gathering up all the tools you think you might ever want to have with you for your particular hobby.
For this Instructable, I'm going to go with a typical sketching and watercolor art type of insert.
Let's see, what will I need:
start with the basics
eraser, pencil, pen
water color paints
rinse cup (2x)
towels for mopping up
extra pencil leads
maybe a small ruler
maybe a ruling pen or two
an extra to loan to that person who will ask for one - one you don’t care if you lose
something to keep your paper in
smarty pants glasses that say leave me alone I’m a serious person and very busy
also they help with closeup detail work
invariably, as much as I plan ahead there will always be that one thing I didn’t bring. We’ll just take that as a given though and carry on.
No doubt some extras will have to be sacrificed based on available space.