1. Hang pictures & artwork: This is key, and can be quite affordable if you frame the pieces yourself. If you don't feel up to the challenge of cutting your own mat board I recommend going to a craft store and buying the pre-cut options and assembling yourself.
2. Install Curtains: Curtains can really warm up a space and provide a nice soft texture if your space is a little more industrial.
3. Secondary lighting: Many small tabletop or floor lamps can really transform a space and make it infinitely more cozy. They can also highlight certain areas or artwork that might have been lost in the shadows.
4. Potted plants or flowers: Always bring a little life into your space, I also strongly suggest you splurge every once in a while and buy a gorgeous bouquet for your apartment.
5. Throw pillows: Another way to add texture, color, and warmth into a room.