Well, do you want it to be an office or a storage room? Some things (like the guitars and books) shouldn't be stored in the basement if it's damp, but is there a way to segregate those items within the room somehow and retain the character of an office, if that is what you want?
We have spare clothing/diapers/baby carrier/sewing supplies storage in our "office" closet, but once that is closed, it's pretty office-like (well, except for the sewing desk...
Do you actually use it/want to use it as an office? I find that aside from schoolwork, my "office" type activities fit better in a secretary near the front door--bill-paying, taxes, and such work better for them than in a back bedroom.
So our office has some bookcases that hold:
Current scholarly/religious reading
School supplies contained in boxes
Office supplies contained in boxes
Old financials contained in boxes
But, with the mix of books and boxes it actually doesn't look bad and is pretty organized.
We have a big desk that we (try to) keep completely empty for special projects--studying for a test, cutting out fabric, etc.
There's a tall computer-style desk with the sewing machine--it has a high shelf that holds two sewing baskets.
There's a set of stacking cube-style cabinets that hold pumping equipment and bottles (not currently in use)
There's a spare comfy chair and a reading lamp. Those are rarely used, but they fit well and make the room look more inviting.
And then there's the closet. It has built in cupboards, which makes it easier, but we have all kinds of frequently-used items in storage there. So, while it's frequently messy, it's also easy to close the doors and not deal with it.
I think we've struck a good balance between the sort of storage that tend sto end up in the "office" and actually having the kind of functionality that we need.
One big step towards keeping it tidy was getting househld finances and mail out of there. All that stuff is confined to a small secretary near the front door. I clean it out fairly often, because it's so small that if I let things go, it gets jammed full of stuff in a week with junk mail and such. I know that if I used the office for bill-paying and opening mail, it would *never* be tidy.
So, yeah, maybe keep the guitars in the closet.
Pay bills elsewhere--kitchen or livingroom.
Sort books nd only keep current ones handy (we have books in boxes, and books in the living room--the office ones are the ones we keep current, bc the office is so prone to clutter)
Use storage containers--honestly plain old cardboard looks fine to me when stuck in between sets of books on a shelf, but maybe that's just me
Keep all spare computer paraphernalia in a box in the basement. That stuff gets out of control too easily to have upstairs. That is imho one of the great benefits of having a basement.
Mostly, decide what you want to do with it. Office, storage, computer room, music room? Pick a reasonable number of functions and go with it. I wanted a sewing room--but that was too cluttery combined with everything else, so it's all confined to the desk in the corner, and losed up int eh closet. A bit more work to get stuff out, no handy cutting table or thread racks, ironing board in the basement. but the lack of visual clutter is worht the extra trouble, since I don't sew that often.
Hope I'm making some sense...