In addition to a carry-on suitcase, I pack a backpack as my airline-designated "personal item." Inside my backpack is where I put everything I might need to pull out at security or access before or during the flight.
I usually pack my laptop, Kindle, and chargers, as well as any snacks (I always bring a bar or two), and my bag of 3.4 ounce (or under) liquids. I put everything I'll need to pull out at security (liquids, food, and laptop) at the top of my backpack so I can quickly pull them out at security and slide them back in after I make it through.
And, since your carry-on will be stowed in the overhead compartment, your backpack should be where you pack everything that you'll need to access during the flight, like headphones, earplugs, and water.