Priority boarding is useless. You wait in line and have to jockey for space. I am usually one of the last people to board, I walk in, go straight to my seat and don’t have to worry about maneuvering past everyone, since they are already seated.
Noise canceling headphones. Pick your flavor it doesn’t really matter but having a nice audio playlist or some podcasts downloaded while not being able to hear everyone else will make the flight so much easier.
You can look up the flight before you book to figure out legroom, as a result I fly jetblue when I can and southwest when I can’t because they average about 2” and 1” more legroom respectively over everyone else.
The smaller and the easier your carry-on is to set down, the better. The goal is for it to be able to fit under the seat and not in the overhead compartment. Overheads fill up quickly, every seat has its own storage. I personally use a small briefcase that I found at a thrift store.
Toiletries, you don’t need as much as you think you need, and they don’t need to be in the carry-on. The only thing I would say needs to be in the carry-on is your toothbrush with some toothpaste already applied just in case. Hotels will typically have something resembling shampoo and body soap. Between those two things it will buy you enough time to get to a store if luggage gets lost.
Luggage is best in wheels and hard cases. I went so far that I use a pelican case as my luggage case. You probably don’t need to go that hard, but if the luggage can be bent with your hand, imagine what would happen if your luggage ended up at the bottom of the pile. Nothing fragile will survive, unless it’s in a hard case. Also if you aren’t confident that your luggage can survive a trip down the stairs, you need to pack it better, luggage handlers are not known to be gentle.
Patience. There’s a lot of hurry up and wait when traveling. Personally I bring a charger in my carry-on so I can use my laptop and phone still. You’re not going to make the plane do its plane things any faster, so just chill out, relax, you’ll get there when you get there. If you do a lot of traveling this is where the lounges come in play, but for one offs not worth it.