Upvote:2
Be aware that Mainz is one of the hot spots for German "Karneval" or "Fastnacht", which basically turns the whole city into Pandemonium for about a week in Feb. See https://www.mainz.de/en/service/carnival-in-mainz.php
Finding accommodation during this time will quite difficult, but crashing parties and passing out on someone's couch is pretty normal (if you have the stomach for it).
Mainz is unusual for a stopover since it doesn't have an airport to speak off. Are you coming by train ?