Simplest thing would be to put a shelf up around the room. You could also put a curtain rod up above it, so curtains could be pulled closed to hide the clothing/etc. that sits on the shelf. Plus, then she has the opportunity to decorate with different fabrics for curtains. Cheap and effective.
Other suggestions could be cabinets, but that is going to get expensive in a hurry, and may not be ideal for clothing. You might want to take a look at Ikea - they are probably the experts at making usable space out of shoebox-sized rooms.
Other suggestions could be cabinets, but that is going to get expensive in a hurry, and may not be ideal for clothing. You might want to take a look at Ikea - they are probably the experts at making usable space out of shoebox-sized rooms.