Everything about this visit was great.
Maria is a super nice lady who made us feel welcome from the start. The room is very clean, and the whole apartment looks like an artist made it. The furniture, the photos, everything looks interesting. It's also in a quiet neighborhood very close to Kreuzberg, and supermarkets are around the corner. When the weather is good, you can also have breakfast outside in the garden.
We had a great stay!
The only minor disadvantage was that there was no S- or U-bahn station closer than a 20-minute walk. But there were buses very close!