Faux Built-In Bookshelves - flank a doorway with identical bookcases that reach almost to the ceiling and install moldings around the tops of the shelves. As a style bonus, paint the bookcase before installation, or line the backs with pretty paper.
11 Painless Ways To Change Up Your Rental (Without Risking Your Security Deposit)
Add lofted storage in an oddly shaped area above the kitchen. If your apartment has high ceilings, utilize the space above your kitchen cabinets to build in extra storage. (Apartment Therapy: 10 Ways to Customize a Rental)