我想問德國的交通

Train should be pretty reasonable choice. Or you can rent a car, to experience
the famous Autobahn, I tried it myself, it is awesome. Driving at 120
miles/hour (not Km) and still get passed by other cars very quickly :)

B&B = Bed and Breakfast, it's a small simple hotel, often run by the
owners. Basically it's like a big house converted into several rooms
to rent out by the owners. Often breakfast is provided (and sometimes
very nice breakfast), hence the name. If you just need a place to sleep,
and want to have more interaction with other travelers/owners,
it's not a bad choice. Could be slightly cheaper compared to regular hotels.

HK booking situation I am not familiar with, leave it to others to answer.
Good luck, and happy travelling. Remember to say "Dunker!" (Thank you
in German)

Rent a car or use train depends on your travel group size. If you are traveling along, better use train. If you have three to four peers. Driving is a good choice. Also judging from your travel plan, driving is a good option. But pls aware that you need to drive a car with manual transmission, because auto transmission car is VERY uncommon in Europe.

If you chose to drive, B&B is a good choice. In Germen, they call them ZIMMERS (which means ROOMS in English). Look for these sign on the road. Ask for price and take a look at the room before you rent it. Price around EUR 30 to 60. If you use train, you can book hotel at Tourist Information Office, ususlly locat at main train station and tyouist attaction location. But they will charge a few EUR per booking. I use booking.com on web to book Europe hotels. They have lots of choice from very luxury to cheap hostel. Price depends on location and class. Usually a double room is around EUR60 to 80 (with private toilet). They need credit card info during booking but they don't charge. In Europe most hotel are small size (around 30 to 50 rooms) but are mostly clean and safe. Some have internet access but mostly don't have something like a gym, swimming pool, business centre (come on, who need that in a Europe vacation??).