You didn't mention if it is your own home or if you are renting?
I would think no matter what surface you will put up on her walls [drywall, paneling or painted plywood as a less expensive alternative], short of tiling the walls, there is always potential for creative damage. I would keep it as simple as possible. If you got drywall [sheetrock] and you don't can't or want to change it, I would use the same paint for the entire room, no fancy two or three tone designs, certainly no wallpaper. It it is your own house and the walls look like mice are having a field day, consider putting 4x8 sheets of inexpensive plywood on the bottom half of the walls and paint it. She'll probably still pick on it, but the actual damage on the walls will be less and it is easily removeable again, when/if she outgrows it. This is also a safe way of covering up electric outlets, if you are concerned about her putting things into them.
For window coverings - roller shades. They are very inexpensive, easy to replace or pop back in if she pulls them out and can be covered with fabric or painted to look more friendly. They stay curled up during the day, and at night come down.
A very plain jane ceiling fixture, which will not cost an arm or a leg to replace.
Simple furniture - a futon matress on the floor if necessary, maybe a beanbag chair [unless you think she'll take it apart], then I would look for a comfortable upholstered second hand living room type chair. She'll need something comfortable to cuddle up and relax, maybe read. I'd keep the furniture as utilitarian and plain as possible, consider second hand if she damages a lot. The more she breaks, the less she gets to have.
If she has electronics, you might want to consider having her "checking them out" library style and returning again after so much time, so it is not a temptation to mess around with when she is by herself. If something gets broken while she has it "checked out", it will not get replaced. Some kids at 11 are still very innocent, others are full in the swing of peer pressure, social life, and worse. I'd say you need to adjust to who she is, or perhaps share some more info on her in order to get better advice.