Disclaimer: I make no claims that I'm an expert in what one might pack in their First Aid Kit; these are the items I've gathered over the years after reading about other people's kits. Some of the items I've used, others I thankfully haven't had to.
Band Aid selection has been updated a couple of times. Note the extra diphenhydramine: I've always made sure to carry this ever since Lily stuck her nose into a wasps nest in the ground on a hike. Her nose/mouth area was beginning to swell and the diphenhydramine I was able to give her helped prevent it from getting worse. (Before doing so, be sure to ask your veterinarian if it's okay to give your dog diphenhydramine and if so, what dosage they would need.) The tick key has also proved a useful tool.
Second Aid, hiking/outdoors edition. The hat and gloves take up a lot of room in the kit and I might want swap out one or the other for more food or hand warmers. I've given this stuff (as well as items from my own First Aid Kit) mostly to people I'm hiking with as opposed to strangers -- which, hey, is a good thing, as that means I haven't run into too many people in need of help so far. It's nice to know that I'd perhaps be able to help a bit if I did, though.
Second Aid, everyday edition. I started carrying this edition more recently and I expect its contents to change as I figure out what's useful vs. what isn't. I added the playing cards last minute -- we'll see how often they get used! I usually do bring a deck on trips. The dog leash and treats are for when I find lost dogs running around, which happens surprisingly often. I only use either of these after a dog has made it clear he's okay with me finding him and bringing him back to wherever he came from, as a dog really has to give you permission before putting a leash on it and treats won't help much if the dog isn't already okay with you. In other situations, treats can sometimes help from the outset: say, if you're at the dog park or at a trailhead and someone's there with their dog and it's time to go but the dog doesn't want to get back into the car because she's having too much fun. Extra hair thingies always seem to be needed. I like the Starbucks VIA coffee -- though I usually triple it up.