If you‘re staying in Neukölln and want a ~50k route without too much traffic, I‘d recommend a loop around Müggelsee via Treptower Park/Plänterwald & Wuhlheide.
Grunewald is sweet as well, but you‘ll have to cross the city to get there. Unless you want to do a longer round via teltow kanal, parforceheide, wannsee, which is a nice single-trail option. If you want to go swimming, Liepnitzsee is a great option but you‘ll have to cross the whole city if you‘re starting from neukölln.
If you‘re staying in Neukölln and want a ~50k route without too much traffic, I‘d recommend a loop around Müggelsee via Treptower Park/Plänterwald & Wuhlheide.
Grunewald is sweet as well, but you‘ll have to cross the city to get there. Unless you want to do a longer round via teltow kanal, parforceheide, wannsee, which is a nice single-trail option. If you want to go swimming, Liepnitzsee is a great option but you‘ll have to cross the whole city if you‘re starting from neukölln.