I thought I could keep this simple:

iPad, Beats headphones, and a couple energy bars.

But then there's the supporting elements... chargers & my outlet splitter for layovers. A book for incase it gets that delayed. Bubble-gum to fend off a bored kid. Sweathshirt if I'm not headed somewhere warm. Trail mix if I skipped a meal or it's that long.

In the end I take the same backpack about anywhere... it's pretty full and while I don't use half the stuff on any one trip it all gets used across a few.