There is really no telling what you will find in there! Also, it probably depends on what bag. In my brush hauler--paint brushes of all sizes and shapes, a deluge of palette knives, postcards for local artists, paper towels, a notebook and pen, pliers, refined linseed oil, turpenoid, a double palette cup.
In my canvas bag--blank canvas and canvas boards from 2x2 all the way up to 16 x 20. I save the larger paintings for my studio, grey palette paper 12 x 16, a hat and sunscreen.
In my French easel--Lukas studio paints set of 12 (20ml) lighter to carry for field studies, a large tube of mixable white, small metal clips, a metal paper towel hanger, palette, more brushes, extra hardware for easel.
Chair, folding chair, rolling crates, tripods, plein air boxes have also been known to make their way into the field with me.
I need to add gloves, wet wipes, and hand cleaner to this list so I'm not covered in paint. Sometimes I roll with an apron, unless I'm feeling free spirited.