If you haven’t sorted out your accommodation by now, you have a problem. All the rooms we had pre-booked through the Mainz tourist board are gone. There seem to be a few rooms left in Wiesbaden, but they are going fast. On the local website, we have listed a some hotels which run their own systems, but most seem to be fully booked, too. Of course, there are sites like expedia, but last time I checked, that looked rather bleak, too. So, let me reiterate: If you haven’t booked a room yet, do it NOW.
If you can’t find something adequate in Mainz or Wiesbaden, Frankfurt, Darmstadt, Rüsselsheim and some of the smaller towns in the area are commutable. But it is important to act now. BTW, as of late, I feel like a travel agent.