Whew, that is wide open what you ask there
If you could give me a better idea of what kinds of things you have chosen to see in other cities, then I could answer better. Nonetheless, I can give some tips:
- Mitte around Hackescher Markt, especially at night, is good for street photography, but keep in mind that it is also touristy.
- The museums and galleries here are top notch. I would pick at least one or two. Perhaps the Egyptian museum in Altes Museum, and the Pergamon or Bode museum. For galleries, the Neue Nationalgallerie normally has good shows, but it can be hard to get in within a reservation, depending on what is on.
- There are some really good photo galleries here, some free, some pay. The Helmut Newton building by Bahnhof Zoo is good, as is Martin-Gropius-Bau and C|O Berlin Postfuhramt. Also try to hit the little galleries around Auguststraße, as well as Camera Work close to Zoo.
- Potsdamer Platz is worth a visit, even though it is flashy and touristy. It is also close to the Casino (never been there), and some culturally oriented buildings like the Kulturforum, and the Neue Nationalgallerie.
- The parks here are great, but definitely at least visit Tiergarten, and exit in the north-east, where you can take in the (lineup likely) free view from the Reichstag and then stroll over to Brandenburger Tor. A stroll along the river afterwards behind the Reichstag is also a likely spot for neat architecture and people photography.
- For younger crowds, Prenzlauer Berg (mid twenties to late forties) and Kreuzberg (a bit rougher, a bit louder, and a bit younger) are the places to go, with Friedrichshein also a candidate.
I am sure I missed something. For those interested in seeing a bit of the city outside the challenge, perhaps we could form a small group of 4-6 people and go around a bit? There is really so much on offer here, I would say that you should try to squeeze an extra day or two in.
Berlin is not larger than Paris or London in what's to see, although the area may be greater. There is hardly anything to see outside the inner ring S-Bahn running around from Westkreuz to Alexanderplatz to Ostkreuz and so on. One exception would be Sanssoucci, a french-style palace with gardens. There are also nice parks and beach areas on the edge, as well as a large in-city forest: Grunewald, all reachable via S-Bahn and U-Bahn. Definitely get an AB train pass for the whole stay. Then you can use all trains, busses and trams without restriction. Make it an ABC and you can also get to the fringes. It is very affordable here. Good food, good times, almost no violence, interesting places, great culture, you could hardly pick a nicer city to visit. Given that I am a non-German who has lived in Copenhagen, Toronto and Montréal, that should say something.