Usually it's my laptop (Lenovo T530 - 15"), headphones (B&O H3), iPad mini, OnePlus 3 phone, and a book. But I've been looking to downsize the laptop because mine is pretty chunky and it's hard to use in coach. And if it's a long trip (transoceanic or deep South America) I'll bring along my Bose QC15s.
I keep all of this in my backpack at my feet.