Had an amazing two night stay at Nena Apartments Metropolpark. The room was simple but had everything you need, even a little kitchenette and a fridge. Bathroom was clean and spacious enough for two people. Staff was very kind and helpful and can be reached over WhatsApp. Great location - very close to the city center, but in a quiet neighborhood with an sbahn and ubahn station just 5 minutes away. Would definitely go there again when in Berlin.