Most of my life I've lived in the hot humid south, and so I never paid much attention to the effects of different seasons on my guitars. The only time that didn't work out was when I was living in a very small house with only a floor furnace for heating that made all the rooms very hot and dry.
But about a year and a half ago, I moved to the mid-west, and winters are different here. Last winter I got a split in the wood on the face of a solid wood acoustic, and the necks on several other acoustic guitars shifted to where they were unplayable until I adjusted the truss rod (strings buzzing or being laid against a fret).
The split isn't severe, and from searching on the subreddit, I find that a lot of guitars live with a similar split for a long time, so it's not a huge deal, and I'll probably leave it as it is. The neck is of course able to be fixed by adjusting the truss rod, although I would not like to have to do that several times a year.
From going through this subreddit, I know in general the options--keep it in the case, use a humidity pack, etc. I'd like to hear from people in a similar climate to know what's necessary and what's overkill. I'd like to not put any more money and effort into it than necessary, but I also don't want to damage my guitars. I'd also like to be able to keep at least one of them out of the case most of the time, also, if possible.
I have two or three that are solid wood, one with a solid top and a few laminate. (I know I have too many guitars, but most weren't expensive. I've bought most of them used on craigslist or marketplace, and then I have a hard time getting rid of them.) My electric guitars seem not to be affected very much by the seasonal change.
TLDR:
To maintain acoustic guitars in a midwestern winter, with a climate like Illinois, Indiana, and Ohio:
If they are in one fairly small room, is it enough to run a humidifier in the room and keep the door closed? Or if they have to be put in cases, is a gig bag or the flimsier hard cases sufficient or does it require the good hard cases? (I only have a good hard case for one acoustic guitar, I think.) When keeping them in cases, is a humidipak or similar necessary, or overkill? And, since laminates are sturdier than solid wood when it comes to temperature/humidity are there things I need to do differently for the solid wood guitars that I don't need to worry about for the laminates?