Germany passport holders do not need visa to Hong Kong for 90 days. But you need one to enter Mainland China, if you enter China mainland first and then go to HK, you need a double entry visa if you want to re-enter China from HK. So, I recommend you visit HK first. Generally, a tourist visa duration is 30 days, but it can be extended twice in China.
There are detail requirements (in Deutsch) on their official web for China visa application. What I can say is that the return ticket and hotel reservation are required when you submit visa application to the Chinese embassy. The application form you can download from their web and fill it beforehand.

Do we need a hotel reservation for the whole time? We want to see Peking, Shanghai and Hongkong and we don't know yet how long we stay where.

Not necessary, Christian. But possibly you can provide the tour itinerary and explaination for accommodation if the visa officers ask for it.

Hi,
If you want to visit China mainland and Hongkong , Macau.YOU can fly to Hongkong and tour there, then take your visa to come to China mainland. The visa with one entry is ok.
If you have been in China, you just can stay 30 days, but wen can help you extend that for another twice, one ce for 30 days.
As to the hotel reservation, you just show a one in China mainland. maybe 5 days or the other days, either is ok.
Any questions about China visa, we can discuss.
Keep in touch!
KR
monica