There are numerous synonyms that can be used to replace the phrase "just plain nuts". Some common alternatives include "crazy", "insane", "mad", "bonkers", "wacky", "batty", "loony", "nuts", "off the rails", "off one's rocker", "out of one's mind", "unhinged", and "whacky". Each of these options can convey a similar sense of disbelief and confusion about a person's behavior or actions. Depending on the context and tone of the conversation, one synonym may be more appropriate than another, but all of these options capture the essence of what it means to be "just plain nuts".