I don't understand why you are not allowed to paint. A simple coat of white paint will freshen just abt any room. Is your landlord mad or something, :)? If painting is out of the question, hang artworks. Framed pictures strategically placed will add punch. Plants are good, devils ivy is pretty and will grow in water or soil and will grow pretty well in regular artificial light. Happy decorating!
Plants are a good way to liven up any bathroom. Some coloured candles, matching towels and Shelves void of clutter. Ensure that the bathroom is always neat and tidy. No wet towels on the floor! also invest in a good shower curtain matching the theme of the bathroom with a nice bath rug. All the best! Hope this helps a bit.Nursery ideas?