I agree with the advice not to go overboard. Try to find things that can do double duty (e.g., a sturdy cedar chest can hold stuff and double as a coffee table). You're not going to live there forever, so don't buy a lot of stuff you're just going to throw away later on.
I would encourage a couple things. To initiate, all of the previous ideas are solid. I would potentially find an antique store/mall and just stroll through it. Keep in mind the things that give you joy and see if you can find some sweet items of antiquity that line up with them. I would also encourage this; my brother and his wife used a pallet as a shelving unit. It was pretty sweet. They found the studs in the wall of their living room, and fastened it into the wall with the bottom against the wall so the 3 runner boards now act as shelves. Happy hunting brother!
Start off by looking in IKEA, and whatever they do, do the opposite.
When you have such a small space, streamlining and efficiency become paramount. But, that doesn't mean it has to be bare or impersonal. Find a few pieces of good quality that genuinely speak to you, and don't include pieces that serve no purpose for your lifestyle. You might want recliners, for example, instead of a formal sofa. And, don't be afraid to be bold.
As for the rest, if you want to designate different spaces within your apartment, you can do so with something on the floor like an area rug. And artwork can be anything, if you even value artwork...some people like the starkness of a bare wall.
Just whatever you do, please, avoid the trap of lining up the furniture against the wall. It's an apartment, not a doctor's waiting room.
Remember the psychological effect of 'priming'. You can take advantage of it by deciding what sort of psychological state best helps you accomplish your goals, for whatever activities you pursue in a given room; and arranging for the decorations that help nudge you towards that state of mind. Or, at the very least, you can avoid adding anything to the room which /detracts/ from that mood.
That said - it's hard to go wrong lining every available vertical surface with bookshelves.
I feel like I should elaborate a tiny bit.
I'm going to move my table in there (it's been my family's dining room table since as long as I can remember...a project of mine is to refinish and repaint it).
Also, I will probably be getting a sofa bed in there so I can have something for guests to sit on, but something I can sleep on at the end of the day.
Ultimately, I'm just looking to set it up as a place to unwind at the end of work while I shine my boots and Sam Browne. I kind of wanted a "theme," like a city or a time period.
At some point, I'm planning on adopting a dog to keep me company (my girlfriend just moved back home, about 400 miles away...fortunately, I have an excuse to go up there at least once a month for drill with my reserve unit)
First thing to do is to sit down in the space think about who you are, and what you do in that space. Use that for your guides. It is your space make it yours.
Add a bit of color to the walls with a painting or an image. Make it classy enough for dates or parents coming by. I like Celtic and Medieval stuff so I would go with some image of castles and such.
If you drink set up a small bar with what you need to make one or two drinks you like. For me it is a bottle of scotch & a bottle of bourbon and 3 bottles of red wine . You don't want to have the party bar, but you do want what you like at hand.
Personally I have a reading corner with a side table for relaxing in my main room. Unlike my couch it does not face the TV.
Beyond that I recommend buying a blue tooth receiver and cheap computer speakers for the kitchen. The speakers will grease and gunk up so you will want to toss the speakers after 2 years or so. They will do the job over the sounds of the stove and fan. The blue tooth receiver lets you run it from your phone.
Have you ever slept on a sofa bed? No matter how well manufactured, it seems like there's always a bar running across the middle that gets uncomfortable during the night. I wouldn't recommend one as your main bed. Not to mention, they're heavy as hell when you want to move them. A futon, on the other hand is very comfortable, and they come in all kinds of styles now-a-days.