The first aid kit in the trunk of the Impala was always well-stocked, out of necessity. When you spend your time getting thrown around by demons, werewolves, and ghosts (among others), injuries become almost a part of daily life.
There was a jumbo bottle of ibuprofen, already half empty. Enough surgical thread and needles to stitch up an army. Rolls upon rolls of gauze and a couple of tubes of antibiotic ointment (Dean had learned the hard way not to just slap on a bandage without any ointment when a cut on his leg developed a nasty infection, scaring Sam, Bobby, and Ellen half to death. Once he had sufficiently recovered and it had been determined that he had narrowly missed full on blood poisoning, he had summarily been yelled at by all three of them. Simultaneously.)
(Later, when he told Jody that story, she yelled at him, too, even though it was years after the fact. He would never admit this to anyone, not even Sam, but it made him feel good, to have someone be maternal towards him after all these years.)
There was an old-school mercury thermometer that looked like it had seen better days. Every once in a while Sam makes noises about replacing it with a newer, digital thermometer, because come on, Dean, that thing's ancient, and you know the digital ones are more accurate anyway. Dean always refuses, saying that the one they have works fine, and a new one isn't worth the expense, especially because neither of them have had a fever high enough to require monitoring in years. Truth be told, it's one of those little things that reminds him of his father that he's reluctant to get rid of. That thermometer was constantly being shoved in their faces as kids, as both boys came down all the usual assortment of childhood illnesses. Croup. Colds and flus. Sammy's constant battles with strep in the winter. Then there was that memorable time when both boys had chicken pox at once, clinging to each other in a futile attempt not to scratch.
Then there was some harder stuff, prescription painkillers that they'd either bought on the black market or acquired from other sources (other sources usually meaning "Bobby"). Unlike the over the counter painkillers, these were barely touched. Things like oxycodone could mess with your head, and to risk that on a hunt would be suicide. No, if something serious was going on, it was better to just hole up in a motel room or at Bobby's until it passed. Unfortunately, knowing there was a safety net if things got bad didn't mean that they actually used that safety net.
(After Bobby….after Bobby, Jody's became their safe house. They hadn't planned on it, but after the boys showed up at her house once with both of them running fevers and pale as death, she made them swear that they'd come to her from now on, if they were too sick or too hurt to hunt. She didn't like the idea of the two of them, sick or hurt and alone in that big old empty bunker in the middle of nowhere. They had protested and mumbled things like didn't want to bother you and you have two teenage girls to look after. She had assured them that it was no bother, and that Claire and Alex were more than capable of taking care of themselves for a few days.)
That first awful day, when Sam had stumbled out of his burning apartment, shaking and coughing, Dean had wrapped a blanket around his shoulders and pulled out that old battered metal first aid kit. The sight had calmed him down then, and it calmed him down now. At this point, the first aid kid was almost as much of a fixture in their lives as the Impala itself.
A first aid kit was a strange thing to get attached to, but Dean and Sam Winchester were strange boys who grew into strange men. And as the Impala rolled on, the first aid kit stayed, rattling around in the trunk, just waiting to be needed.