I feel like a sheep, but I'm avoiding writing a paper here, okay. :/ Also it seems useful.
"Comment and we'll work out what we can assume that our characters do together offscreen! We can even assume our characters have seen each other around, even if they haven't met onscreen, if it seems logical."
Hanajima mostly hangs out in Girls' 1 (where she plays card games on her laptop, and probably beats the AI every time) or the library (where she reads trashy novels), or the Mess Hall if there's any chance of there being food there.
Hyatt... spends a significant portion of her offscreen time in a living-impaired state, but when not dead, she can often be found at the ACROSS base. She also loiters around the hospital a fair amount, for no discernible reason.
Hisui works at the Mess Hall (and thus is likely to have at least seen a lot of people offscreen that she hasn't met onscreen, although she probably hasn't spoken to them). Otherwise... well, she spends a certain amount of time each day tidying her own cabin, and probably hangs around Shiki-sama's teahouse sometimes.
Momoko mostly spends her time in the Arts & Crafts Hut (doing embroidery or designing outfits) or the library. She also spends a fair amount of time on the computer -- not too much, because she's not a nerd or anything (perish the thought), but she uses it to watch TV shows and window-shop for clothes she can't buy because the orders never come through and things like that.
Maria practices daily at the shooting range and probably does some general training-type stuff as well -- I don't know, runs around camp or something. She also enjoys, for certain values of "enjoys" where it doesn't seem like she's actually having fun, spending time in the library reading Serious Books.
Sara spends a lot of time in various indoor areas on the computer, doing secret hacker-y things. Or just, you know, surfing the internet. Unlike the others, though, she does make an effort to get out and explore a bit.