I would keep it simple- start with one (or more) of those LED taplights or an LED lantern (that are battery operated) for light.
Next, throw in a papasan chair and/or bean bag seats (depending on what you already have or can get and what you like as well as how much space is in the room).
You can paint the room- be sure to keep your bedroom well ventilated during the process and leave the closet and room open until the paint is dry. Hello Kitty makes me think of white and pink and red. You could do all of the walls one color or just paint one to have an accent wall or whatever you want. Another option would be to use paint that has a chalkboard effect or magnetic (so you could use magnets to stick pictures, notes, etc. to the wall). There is also glow in the dark paint, but I do not think I would paint the entire room with that, but you could use it to write your name or other things on the wall.
I would just add items as you find them, as long as you have space. Try to avoid electronics unless it has been properly wired (you do not want to cause a fire). If it has been fitted for electricity, you might consider a mini fridge, a radio/CD player that you can hook your iPod up to, a television, etc.
Personally, I think I would add a bookshelf with books such as Nancy Drew and Hardy Boys and other "mystery" books that mention secret rooms and things of the sort.
