Honestly, the one thing people never mention enough: bring something for wind-like, wind you will write poems about later. Even in late summer, those summits will have you questioning your life choices in a t-shirt. Get a good shell, even if it feels overkill at the trailhead.
As for water, don’t count on refills; unless rain puddles are your thing, haul at least 2-3 liters. If you run dry and cross a stream, filter it-unless you’re feeling lucky (don’t). Snack-wise: plan as if you’ll burn calories just from the views, because, trust me, you will.
And above all, pack a sense of humor. You’ll need it when you realize Mount Washington’s “last little bit” is code for “I hope you like stairmasters.”