Sounds like a new paint job would definitely help, and if you're not allowed to replace the larger fixtures like the sink or vanity, how about replacing some of the smaller fixtures like the faucets, towel bars, etc? Sometimes little updates like these can go a long way.
You could paint before you do the windows and then do touch-ups after. If you're careful and you plan ahead it shouldn't be too much work, but indeed you should absolutely do floors last.
I agree with TheClumsyCarpenter. Even if you're not planning on selling anytime soon, it sounds like you're looking for something that won't cut the opening off entirely. How about some sort of portable pantry or shelving space...