Flea Market Sunday 09:00-18:00 at Mauer Park in Prenzlauer Berg district
As James mentioned, right next door to FixFest venue is C/O Berlin they have exhibition by Robert Frank, seminal 20th century photographer who sadly died earlier this week aged 94.
En route to Hackathon in Mozilla, if you go via Berlin Ostbahnhof, there is East Side Gallery (old Berlin Wall covered in murals), plus on West Bank. also in the area is Treptower Park with Soviet War Memorial.
Museum Island from Humboldt University towards Alexanderplatz, with side detour to Hackescher Markt,
Lots of bars / clubs from Warschauer Strasse up towards Boxhagener. also a few decent restaurants that are not too expensive.
Other good places to just wander around in Neukölln,
Coffee shops - Godshot and No Fire No Glory in Prenzlauer, The Barn near Kopenplatz and more importantly nearer the hotel and venue is The Barn on Joachimstahler.
I’ll add more, after asking my son who lived there for 2 years…