You could effectively show up naked and with zero possessions on I-day and be okay, but since most people don't want to do that for some reason...
On I-Day: Keep in mind this is a bag you will drag around for half the day then toss into a storage room and essentially never see for the next six weeks aside from the stuff they will let you keep in your room. Small backpack with at most a couple extra pairs of underwear/bras, compression shorts (issued ones aren't that good), paperwork if you need it, cell-phone and charger in separate bag labelled with your name, and some tampons.* If you wear contacts/glasses, wear your civilian glasses on I-Day and keep a set of contacts with plenty of solution in your bag along with an extra pair of civvie glasses. Usually they have BCGs ready to go for you, but if they don't you have to wear what you brought (and no contacts until PPW).
*Without grossing the dudes out too much: it depends on you whether or not you'll need them and you do get issued some, but if you have a brand/style you like I'd bring them.
Care packages: Gatorade packets or other juice mixes are CLUTCH. You can't put them in your canteen, but you get issued a nalgene and having something to sip on at night that's not water can be a great feeling.
Since the plebes no longer get box juices in King Hall (which is a travesty, but that's another story) and are basically force-fed Gatorade ice tea mixes or something similar might be better than just Gatorade mix.
Mini Clif bars and other small snacks that aren't (completely) junk food are good too. Candy is nice, but it's just sugar and doesn't do a whole lot for you. Think foods that don't make crumbs, break easily, get caught in teeth, etc.
Non-food items are really not necessary at all. Plebes get issued tide pens/shout wipes/all that crap. Hold out on sending non-food items until the first phone call and then decide what you want/need.