Depends on your budget. Art can make a large impact, adding a rug, pillows, something impactful like a screen behind the couch, or a bookcase filled with interesting pieces. Go antique hunting, accessorize your coffee table, slipcover a chair, paint or distress some furniture and accessorizing will do the trick. Look for inspiration on Houzz photos (I typed in small apartments for the photo below) or in magazines.

Curtains might help.

Maureen's art; er612's window treatments and then add mirrors. Try IKEA and get several. They will reflect the art, the drapes and brighten your space.

And two favorite solutions for renters:
1. Removable decals for the walls.
2. Use liquid starch diluted with equal parts water, then dip a fabric of choice in it and apply to the wall like wall paper. It sticks as long as you want it to, then removes easily leaving no damage behind.

Thank you for the help! these are all great ideas to explore