Decorating your apartment is hard to do especially when you can't paint walls and you can't put nails in the wall. Here are some ways that you can make your apartment not so plain, WITHOUT getting in trouble with the owner.
Since you can't paint the walls why not try painting the furniture. Get some cheap furniture at a yard sale or a goodwill. Paint, spray paint, hot glue designs that is your own style. This will add color to a bland room.F
Add cool,fun curtains. Get a glue gun and some fabric and add crazy designs. Use vibrant colors or patterns. But remember when washing your curtains, only hand wash because your only using glue and the curtains wont hold together when thrown in the washer machine.
Vary the height of your furniture. Mix taller pieces with shorter pieces. This adds interest and uses some white space up.
Try using a wall decal. Instead of painting the wall, stick these decorations on. It's fairly simple. Wall decals are made to be applied and removed without any damage to the wall. You can buy this at a dollar store easily but i would not recommend buying it there. Try looking for wall decals online or at an Ikea because this will have better quality and a broad selection.
Try using color clings. These are cheap and fun. Color clings stick to windows without leaving any marks. Use color clings to decorate for holidays or any occasion.
Try an area rug. This is another great way to add color and texture to any room. If this is too expensive for you, you can always make your own by using old blankets or jeans or just scraps of materials.
You can also use Command hooks. They are plastic hooks that stick to your wall by a special adhesive strip that is removable. With this you can hang up pictures, paintings, clocks, etc. There are 3 sizes, rated at 1, 3 or 5 lbs. No holes in wall is the key feature.