How do you kill 30 odd hours strapped into the seat of an airplane? Movies on a tiny screen with the background drone of jet engines are OK for a while, reading is fine for a couple of hours, but scribbling in a little sketch book eats up the hours and is a lot of fun.
To keep things simple I use a tube of black and a tube of white gouache, a small brush with a built in water reservoir, a charcoal pencil, glue stick, fine line black pen and a liner brush cut in half.
I once tried a uni posca white paint pen, but the change in atmospheric pressure at 30,000 feet caused it to empty half a cup of white paint all over my sketch book on the first stroke. The Uni Posca is only used at ground level now.