I had a green plastic box that I filled myself when a hockey goalie (so carrying massive bag already). Most useful items were : Paracetemol tablets (lots of late teens, early 20s girls on Saturday mornings!!), plasters (ordinary ones), elastic strapping for sprains, and a make your own plaster kit (roll of lint, roll of tape and a scissors) for large or unusual cuts. Steristrips handy too, and Savlon spray and ointment.
Nowadays, I have myself set up differently. Home and cub scout uses. So lots and lots of plasters, steristrips and my MYO plaster kit. (Lots of penknife cuts doing whittling). Savlon. Couple of elastic bandages, sling, etc. Wound wash and savlon wipes. Gloves, cotton wool pads (just unused makeup ones in a Ziploc bag), safety pins, packet of tissues, roll of sandwich bags for waste (often up a mountain with no easy access to bins immediately). Tings at home include burn ointment, big gauze pads, nail covering gauze kit. Anti-bug stuff, suncream and aftersun. And assorted meds to suit most needs - paracetemol, solpadeine, upset tummy, diarrhea, dry and chesty coughs, sinuses, antihistamines, rescue remedy and those specific to our own needs.
Am a trained first aider as well, and keep my manual in the green box, an older manual stays in the big box at home.