Top idea Kit_e. I bought the St. John's ones for both cars, one for the house and one for the boat. (The GLC one's are HEAPS cheaper and look like they have just as much stuff). I also have different stuff in each one.
Given the base level of knowledge that most of us have, I think it's a good idea to unpack and repack every first aid kit. The kit would be useless to you if you pulled it out the first time, didn't know where anything was and didn't know what anything was for.
For the boat kit, I added sea sick tabs and ginger pills, hydration packs, more sunscreen and blankets. For the wife's car, I added a warning triangle so that you can put it in the road in case of stopping to assist at an accident. In the Prado's one, I added sea sick tabs, sunscreen and hydration packs too, as well as a few extra instant cold packs. And in the house one, I also added the instant cold packs.
There are 6 things I add to every FAK as well.
Condoms x 6 (No, not what you are thinking
). Get the non-lubed. These are great for covering protruding foreign objects or carrying water. They're also good if someone loses a finger or toe. Keeps them clean in transit, if necessary)
Tampons - Great for stopping bleeding in really large wounds or plugging a slow leak (boat of camper)
Donald Duck plasters - already mentioned and absolutely critical
Small
LED torch and extra batteries - disaster never strikes when it's nice and sunny and in case you need to see into a wound or in a mouth (or to get a buzzing bug out of an ear.
Lighter or matches - in case the torch doesn't work or you need heat for whatever reason.
Sharpie write anywhere texter. It is impossible to label the skin of someone's arm (if you need to) with a pencil
Also, because we are generally adventurers and head out of civilisation, let me share two military-style tips with you for arranging your FAK and using it in the "wild".
First and foremost - go for a quick run. Just 1km or even only 100m, but run hard so that you are out of breath, then immediately open your FAK and get to everything. (i.e. unpack it). If the mud hits the fan in the bush or at the campsite, you will need to get into your kit when you are absolutely wired with adrenaline. (Some of us are wired for fight and some for flight, but all of us can function with a little training). If you then find that the kit is hard to navigate - get a different bag. The first one I bought from St. Johns is now my bag of 4x4 bits for this reason and has been replaced by a hard box-type kit and I can get to the thermal blankets at the bottom of the kit in just 5 secs.
I would also suggest that you unpack your FAK and do 2 things: When you get it and write down the expiry dates of things like Panadol and eye flushes and put a label on the outside of the kit (Easy if it's a box-kind, or use a spare sheet of material and the Sharpie to label the other kind. (This makes it easier to replace expired stuff and easier to check the kit before each trip). The second thing is very important: If you unpack something and you don't know what it would be used for or how to use it - throw it away or learn from a professional what it is used for.