I suggest you take your bag of stuff (i'll give you the contents below) and go out in the yard behind your house and paint something. anything. you'll soon find what you really need, then go in the house, throw that thing in the bag and continue painting, then leave all that stuff in the bag and never take it out. no more leaving crucial stuff in the car or at home when you've hiked two miles across the dunes in 95 degree heat!
So here it is....
Pictured above is what i carry when i want to carry the least amount of gear, First grab a medium sized travel bag or better yet a backpack. throw in a cheap metal tripod easel, and some of those big clamps you can get at Home Depot (they come in so handy), then grab a size 2,4, and 8 flat synthetic brushes and a liner (also called a rigger), a small palette knife, plastic tub to put thinner in to wash your brushes out with, some little clips from the office supply store to clip it your box, and a gatorade plastic jar for thinner (hasnt leaked on me so far!). A travel size shampoo bottle of Liquin (my medium of choice), magenta stick of oil pastel to draw with, and yellow ochre, and orange tubes of acrylic paint (if you like acrylic underpainting), paper towels and bug spray. you'll need some panels to paint on (i suggest 6x8 and 8x10 and if you dont want to carry wet paintings back to the house, you'll want a "wet box" which keeps the paintings from touching and smearing. a pochade box (a cigar box will work too) with paint in it. in my case... magenta, yellow ochre, cad yellow med, cad red light, alizarin crimson, turquoise, ultramarine blue, orange, sap green, and cobalt violet around the edges of the box. i made this ultralight painting box out of one of those aluminum forms holders you get at Staples Office Supply. works great. i clamp it to the easel with the big orange clamp you see there. a painting umbrella is very useful too so i make room for it as its the same length as the tripod and weighs hardly anything.
Sounds like a lot but it all fits very easily into a small to medium travel bag. if i'm paddleboarding i stick all this in a platic trash bag to keep it dry.
below, i've got my paddleboard ready for a plein air painting trip up and down the marshes at topsail island.