Grand Valley State University - John C Kennedy Hall of Engineering

If you go in the door facing Front St, just go straight through the open area until you're in a hallway, it'll be on your left. There is an entrance/exit along the path that connects the outside area to Fulton, which is directly next to the restroom too - if you need to go straight in without encountering people there's a high success rate for that. Easily the best neutral downtown bathroom for introverts

There is a desk but if you walk with purpose they won't question it. Closed during typical college breaks, open for summer classes for parts of the summer.