I leave for basic dec 15, and the general consensus is a duffel bag like this
https://www.amazon.com/gp/product/B07GSJ45CF/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
you don't want to try to wear 2 backpacks on your back, and suitcases aren't as easy to throw around like we'll be doing in BMT.
And yea I'll be COMPSEC apprentice (cyber surety), people recommend just waiting to buy a laptop once you're at tech school.
also people recommend a lint roller, extra compression shorts/underwear (closer to 7 pairs instead of the recommended 3)
I know about the luggage bag: you'll definitely want a gym bag. Something like this, in black.
For everything else I'd pack for the worst, hope for the best. Whatever you pack will eventually get locked away until BMT is over apart from things like money, envelopes, ID and stamps.
The quarters is a good idea because every bill that you have in your locker or on you has to be accounted for with a money list that has each bill's serial number (and you need to handwrite multiple copies of that list). I'd actually suggest half-dollars if you can get them but quarters also circumvents this hassle.
To strap on the bike? Quick amazon search, no brand and no frills but if you have a rear seat on the bike and a couple of rock straps, you're golden.