Note that no surcharge for luggage is something specific to Frankfurt, not to Germany. In Munich, for example, it's 60 Cents per piece of luggage.

To have a safe estimate. As with the example of a trade fair happening in town, I would rather sleep at the airport than pay 500 Euros for a night. If the airline pays, however, I will gladly take that offer.

To anyone who reads this in 2017, with the refugee situation, there are regular border controls again, e.g. at the Austrian-German border. You still will most likely be waved through in a passenger car with German plates, though.

Multiple uses should not be the issue, the ticket get marked as "used" immediately in the system. It's more about preventing unauthorized people from reselling these tickets having used stolen payment data to buy them.

You could test it next time by putting a small dot inside the label with a pen. This wouldn't be spotted (and replaced for that reason alone), but it makes your towels distinguishable.

Just fyi, I cannot see any free seats on the 8:14 train, neither first nor second class. @Tor-EinarJarnbjo , are you maybe logged in to that website and getting benefits from their rewards program?

You may also consider congstar, a subsidiary of Deutsche Telekom using the same network, but usually for cheaper rates.

Some smaller ones are also more likely to try making you pay for existing damage when returning the car. Not all of them for sure, but better take photos of every aspect of the car before using it and have the employee note even small scratches. (Do that for the well-known brands as well, but even more urgently for smaller ones)

Your comment about being wary with Marriott is important. I stayed at the Marriott in Sindelfingen / Stuttgart last week, and while there was a desk, it was so small that working on it would be no fun at all. The size is about what you would find in front of you in a lecture auditorium. Enough to put down a book, but terrible if you have several of them and a couple of notebooks.