Dress code is pretty much sweat pants (or any loose pants) and a baggy tee shirt, you will probably not want jeans as they will ride up your body and constrict your legs as you sit in those really comfortable for torture equipment chairs the padding has been compressed over 20 years ago ... and have to keep shifting to get your weight off your tailbone.
Furthermore on domestic US-Air flights even in first there will be no video or power ports so bring your own entertainment, there is a possibility they will have internet you can pre-pay $14 for the whole day before travel. I don't know if the flight from PHX to HI will be any better (their websites says there is video but that is subject to change without notice so be prepared) the 752 (757 v1) to HNL the seats will be old (The 757 has ceased production in 2004 but us-air is from late 1980s) the video if there is some will be a monitor in the center. There is essentially no power (even thought it says there is) because it is an empower connection that requires a special adapter (that of course the airlines doesn't have but you can buy duty free if you choose). As for seats get row 1 the recline in first class on the 757 will place the seat in your face in row 2. On the plus side they make a pretty good Mojito (free of charge), you will get fed, and they have a nice selection of snacks in first class.