Official Checklist: for comparison.
Summary and take-away follows.
- Appointment (alternative link) printed out.
- Printed, filled out but unsigned Form DS-11 (A4 paper is ok)
- Social Security Number
- Pre-paid, filled out courier envelope: DHL Express Easy Brief
- Evidence of U.S. Citizenship (e.g. expired passport)
- Present Identification (e.g. German Personalausweis)
- Photocopy of Identification (I was not asked for this)
- Photo (2 x 2 inches - 51 x 51 mm)
- Around $135 dollars in cash or a credit card
Do NOT bring any electronic devices. They are 100 % strict about this.
There is a little shop ~500 meters next to consulate which takes the mobile phone in safe keeping for €2 in case you forget. You get a numbered plastic card to get the phone back later.
The appointment is "only" for the front gate. You will get a number from the front gate and later inside they will call your number. Waiting time inside is up to and around 45 minutes.
Present Identification: This one is interesting. The official checklist states that an expired passport would be sufficient. But in my case they wanted another ID so I used my German Personalausweis.