My wife and I are spending a week in Ireland (for her) and then a week in Germany (for me) in early May. I want to see places I saw 45 years ago while stationed near Frankfurt, including the Berchtesgaden area, and then spend a little time on the Mosel and/or Rhine Rivers (with numerous side trips). We have intended to travel by car for both legs of our trip and my Wife has all of our accommodations pre-booked for our (mostly southern/mid-west coast) Ireland trip. I prefer being able to adjust the itinerary, especially with such an ambitious schedule and not knowing what the weather will be like. When I last spent time in Germany, finding a Zimmer or Gasthaus was easy, inexpensive and enjoyable. We have the tools (Thanks, Rick!) to enable scheduling a day or two in advance, once plans firm up. My wife is very uncomfortable with the idea "playing it by ear," understandable as she has never been out of country. Any suggestions?
In May, I'd wing it - especially since you'll have a car.
There are websites where you can find accommodations and book, most notably the individual town websites, www. "town_name" .de. Others, who know about this, will be looking at these towns, and taking the best places. If you wait, even in slow times, you'll get what they didn't want. Yes, you can drive around looking for a place, but you will use time and gas, and won't necessarily get the best place.