Cannot second this enough! Low quarters are SUPER important to break in - your new cadet will wear them for about the first week of Beast incessantly! Super thankful I broke mine in REALLY well - several miles of walks to break mine in. Same for running shoes - Brooks are super popular for Beast and at USMA in general, and mine treated me wonderfully, but make sure you put some mileage on those too, otherwise they'll be in a world of hurt. For boots, anything that falls under the rules of AR670-1 is allowed - some common boots are the Garmont T8s and the Belleville Khybers, but I personally like the Danner boots. I would also recommend a G-shock watch (or something nice and waterproof that has at least one alarm and a stopwatch/timer) and a Leatherman for the multi-tool.
If you have any questions specifically on the cadet experience, especially as a plebe, I'd be happy to help! And frequent the Facebook parent pages - they helped my family out a TON in terms of prep before R-Day!
Good luck to you and your future new cadet!