Here's what I know -- I'm sure a parent or more recent mid can elaborate.
Most days start with reveille at 0545. You then have PEP from 0600-0730. That is followed by quick shower and then morning meal formation and morning meal. The morning is filled with a series of evolutions that could include: marching, swimming, honor lectures, validation tests, sailing, damage control, obstacle course, shooting, confidence course, getting measured for uniforms, etc. You then have noon meal formation and noon meal. The afternoon is filled with more evolutions of the type listed above. There is evening meal formation and evening meal. In the middle of this are chow calls, rates, room cleaning, watch, parades, and I'm sure other stuff. Lights out is probably around 2130.
You get Sunday morning off. In my da, they let you "sleep in" until 0745. Many mids choose to attend chapel (you can only go in the yard during the summer) but it's not required. I can't remember what we did Sunday afternoons; I don't think we had yard liberty but also don't remember really aggressive activities. Hopefully, someone else can enlighten me.
There were some "fun" events such as bonfires, the obligatory Orioles baseball game (we didn't get it b/c it was a strike year!), swimcalls (swimming for fun), and I'm sure a few things I've forgotten.
Honestly, you can't really "prepare." You'll have way more to do than you can possibly do, but that's sort of the point.