An expended scale voltmeter (the kind that measures the battery under load). THAT will tell you when your flying day is done.

After run oil. A dish-rag or golf crying towel (won't scratch coverings and much nicer to wipe your hands on or cover the carb. in snow/rain). Insect repellant. Spare servo screws, bushings and grommets. A pen/pencil and notepad (when you get old your memory will fade). A finger prop balancer. A toothbrush or small soft-wire brush. A 3/32" or 5/64" finger drill.

A quarter or pre-paid phone card. A bottle of water (I helped a guy who shot himself in the eyes with fuel the first time he used his new electric fuel pump - and you might get thirsty, too).