Two items that I wish I had a few times early on, items that I believe we now stock in ours, are the cold packs you break to turn cold, and bandages that come with a gel to help cool and heal burns.
True story - was running our hydroseeder and crawling around it when I accidentally place my hand firmly on the muffler cover. After a couple minutes I saw the shape of embossed warning metal warning in my hand; a hand with a circle around it and line through it.
It was like I was that German guy in Indiana Jones grabbing the hot medallion and burning it's shape into my hand. Felt pretty stupid to have the "no hands" warning burned into my hand.
Later that day I tried my first burn gel bandage - really worked well.
As I think about it, a couple other items come to mind - tongue depressers or popsicle sticks to tape up injured fingers, tweezers to pull the various things we get caught under our skin, rubbing alcohol, and maybe aspirin or other pain killer.