My favorite is also the in-ear noise canceling pick from Wirecutter: the Bose QuietComfort 20. They’re small, light, and you can somewhat-comfortably sleep with them on (at least, better than many other options). Most importantly, they reduce an incredible amount of noise. If over-ear headphones are more your speed, Wirecutter suggests the Bose QuietComfort 35 Series II.
You could, if you want to go the really budget route, just use earplugs (which many airlines provide in amenity kits). However, those rely on getting a good seal between the plugs and your ear canals which, on some people (like myself), isn’t easy. Plus, even if you get a good seal, the sound of an aircraft engine probably isn’t going to be reduced as well as it would be with a good pair of noise canceling headphones.
I’ve never been on a flight that had a steady or predictable temperature. It’s pretty much impossible, given the cold air outside, the humid, heat-producing mammals inside, and that pesky ball of fusion in the sky (or the lack thereof, depending on the time and weather). Layers are key. Some airlines will give you a threadbare blanket to “warm” yourself if it gets really cold, but I’ve never regretted bringing a hoodie or pullover. Worst case, it doubles as a pillow. I’ve had good luck with Smartwool outer layers, but they’re expensive and really anything easily removable will work here. I also tend to never wear shorts since I’ve been cold on planes more regularly than I’ve been hot, but you know your body best.
Different parts of the plane will also have some effect on the temperature. Window seats are likely cooler than aisles, and exit rows cooler than all others.
Anything you need for your personal comfort
There is a massive market for in-flight comfort products. Most are pretty useless. For example, if you only fly once or twice a year, spending money on a travel pillow probably isn’t worth it. Before you’re tempted to spend money on things you’ll use once or twice, consider saving that money for a seat upgrade.
However, other things may be good to pack in your carry-on. Most airlines on long, overnight flights, will also give you a sleep mask and a small tube of moisturizer, both of which you might find handy to bring yourself if you have a mask or brand of lotion you prefer. Airplane air gets very dry, and even at night there are lights around the cabin you might find distracting.
Usually you’ll get a bottle of water for free, but if you’re the thirsty type, bring an empty water bottle and fill it before you board. A snack won’t hurt either; just keep in mind that many countries, especially smaller island types, won’t let you enter with fresh fruit.