NIE
This is the easy bit oddly enough, given the fuss that is made of it.
My wife's work got the NIE sorted out for her but this proved to be a quick and easy process at Barcelonetta police station. Where you had to queue with a form filled in to say what work you did and the proof of address paper from earlier (naturally plus passport, photos et al).
This was actually the simplest of all the tasks we did.