Seems that answers above come from people around the world. Let me take in local people’s view.

You’re from germany, right? So you’re westerner or called “Farang” here. Every farang I’ve ever known prefer “Khao San Road”. The nearby area is old city, which you can go to the grand palace and temples in the opposite side of the river.

The rest depends on your preference.

If you would like to see temple, palace and musuem. I think Tourism Authority (http://www.tat.or.th/) will have a clue. Most important thing is dressing polite, or you won’t be allowed to get inside.

If you love shopping, Chatuchak Market can be reach by the skytrain. It opens on Sat and Sun only. Department store area is Siam (skytrain as well), which there are 6-7 huge stores locate in row!

For the nightlife, Khao Sarn itself has some. Foreign guys will love Phatphong and Sukhumvit area. But local people don’t go there (too expensive), there are some nightlife area around.