I'm already a working EMT, and have been for over 25 years. I used to be good about carrying a "Randy Rescue" kit in my truck, but have slacked at replacing gear when it degrades or expires. Definitely something I should be giving some more thought to...
When I did, it included a tourniquet, triangular bandages, CPR mask, blood-pressure cuff and stethoscope, various trauma dressings and bandages, some Kling, OP airways, and gloves. I guess if I put it back together, I'll probably add a quick-clotting agent.
When I used to ride with an off-road club, I had that kit, plus various-sized splinting boards and a spineboard with straps on board as well. That was quite a long time back, maybe 1992 or so.