You need to go through your rental agreement with a fine tooth comb, before making any irreversible changes to your property. Some landlords will allow you to wallpaper or paint the walls, as long as it will be viewed as an improvement or if you agree to repaint or paper it in its original state at the end of your lease. Decals or wall stickers that peel off without a trace can be the ideal alternative.
When landlords dress their properties in preparation for rent, they tend to clothe their floors in hardwearing vinyls, engineered wood and neutral carpets. So warm and brighten up bland floors with a rug. The brighter it is and the more irregular the shape, the more individual your room will look. And it's not just living rooms that will benefit from rugs, they work just as well in bedrooms and hallways too.
You don't have to live with everything your landlord leaves you. Pack away any curtains or blinds that were up when you moved in and replace them for the period of your occupation with some that are more to your taste. Switch nasty nets with more stylish window film, think about putting on your own toilet seat and shower curtain and you don't need to live with their lampshades either.
For the final touch, jazz up the furniture. Use cushions and throws to spruce up sofas and extra pillows and a bedspread to smarten up the beds. Then really make the place your own by filling the shelves and window sills with your pretty bits and pieces: books, vases, mirrors, photo frames and other treasured mementos.