Berlin's main sights are free, outdoors and about the twentieth century. They are better absorbed alone, at your own pace, without conversation.
The free sites that matter
The Berlin Wall Memorial on Bernauer Strasse — a preserved section with the death strip, a watchtower and documentation of what the border system actually was. Free, outdoors, and considerably more affecting than the alternatives.
The Topography of Terror — a documentation centre on the site of the Gestapo and SS headquarters, with an outdoor exhibition along a surviving Wall section. The clearest account of how the apparatus worked.
The Memorial to the Murdered Jews of Europe — 2,711 concrete stelae on undulating ground, with an information centre beneath.
Tempelhof — a decommissioned airport whose runways are now a public park.
The East Side Gallery, a painted stretch along the Spree, which is a photograph rather than an explanation.
The Reichstag
Free advance registration required, with slots released weeks ahead and filling up.
A same-day registration office nearby handles remaining slots.
The glass dome above the parliamentary chamber has a spiral ramp and an audio guide, and it is one of the better free things in any European capital.
Museum Island
Five museums on an island in the Spree.
The Pergamon has been undergoing extensive renovation with sections closed — check current status before planning around it.
The Neues Museum holds the bust of Nefertiti.
A day ticket covers the island and beats single entries.
The districts
Berlin is very large and the sights are spread. Group each day by district, allowing 20–30 minutes of transit between areas.
Kreuzberg — Turkish Berlin, the canal, street art, and the bars.
Neukölln, adjacent and younger.
Prenzlauer Berg — the East's gentrified quarter, with Mauerpark's Sunday flea market and public karaoke, which is free and genuinely good.
Friedrichshain — Karl-Marx-Allee's Stalinist boulevard and the club scene.
Four days
Day one: Brandenburg Gate, Holocaust Memorial, Topography of Terror, Museum Island. Evening in Kreuzberg.
Day two: Bernauer Strasse in the morning, then Prenzlauer Berg. Reichstag if registered.
Day three: Tempelhof, Neukölln, and the East Side Gallery.
Day four: a museum, or Sachsenhausen memorial, or Potsdam's palaces, both reachable by train.
Meeting people
Hostels, which Berlin has in quantity and at low prices.
Free walking tours, which are unusually good here given the material.
Mauerpark on Sundays — the flea market and the karaoke, which anyone can join.
Language exchange evenings, common across the city.
Practical
Cash is still expected in many bars and smaller restaurants, which surprises visitors.
Sunday closes most shops, and is when the parks and markets fill.
The S-Bahn and U-Bahn are excellent and a day pass is cheap.
Club door policies are genuinely selective and being turned away is normal rather than personal.