Some more tips:

February is a good time for nightly walks. At night, Berlin has a special charme. Many of the buildings are illuminated, and nightlife is pulsating.

At night, walk Friedrichstraße in northern direction beyond the train station, then right into the Oranienburger Straße. On the right side, you will find the Tacheles. It is, in fact, a ruin from World War II (it used to be a Jewish department store) which has been turned into a nightlife spot for the alternative scene. Visit also the art galleries inside (they are open until past midnight).

http://super.tacheles.de/cms/

Stroll along the Oranienburger Straße, maybe diving into some of the subterranean bars. Or eat a Currywurst at the streetstand. Go past the synagogue (you may visit during daytime).

Better do not chat with one of the ladies standing at the curbside (they speak only Russian and a few words in German which you find in no dictionary). At the end of the Oranienburger Straße, go left into the courtyard (Hackesche Höfe, with art nouveau facades).

http://www.hackesche-hoefe.com/index.php?id=32#
(sorry, this website is in German, but you may see the pictures on the virtual visit, just click on the items on the map)

Inside the first courtyard, there is a theater called "Chamäleon". They have an interesting show, even if you do not understand German, you might enjoy. Much of the show is nonverbal. They even have a website in English:

http://www.chamaeleonberlin.de/logik...etlanguage=eng

If you leave the Hackesche Höfe you are at the Hackescher Markt. You may go into one of the bars which are under the S-Bahn bridge.

Other walks at night include:

- Unter den Linden, the section between Museum Island and Reichtstag

- Gendarmenmarkt

- Potsdamer Platz

- Kurfürstendamm (in West Berlin), including some side streets

Here my favorite museums:

- Pergamon Museum. This is absolutely outstanding. The Gate of Babylon (original), a whole desert palace, the Pergamon altar, and and and
- Egyptian Museum (Altes Museum), with Nefertiti's bust
- Helmut Newton Foundation (this might be something for your boy-friend)
- Jewish Museum
- Museum of Natural History/Museum für Naturkunde (the world's largest dinosaur skeleton)
- Museum of Technology/Deutsches Technikmuseum
- And dozens of world-class art museums: Sammlung Berggruen (for Picasso), Alte Nationagalerie (19th century), Neue Nationalgalerie (modern art), Flick Collection (art of the last 20 years)...

http://www.smb.spk-berlin.de/smb/sam...mp;p=3&n=1
http://www.museum.hu-berlin.de/index_english.html
http://www.dtmb.de/index_en.html
http://www.berlin.de/museumsfuehrer.en/index.html

Thank you so much everyone!

You have gotten some excellent advice here, Saira. I'll add just a bit. We loved exploring Prenzlauerberg - just north of Hachesche Market. There are lots of great shops, a cultural center, and some good restaurants in this area. We saw tons of families and young people.

A few additional museums:
- the Gemaldegalerie has a fabulous collection of medieval and old masters.
- we really enjoyed the Berlin Gallery. It was a little hard to find, but is only a few blocks from the Jewish Museum

I'll include links for a bunch of trip reports for Berlin that I found when I was doing my research.

PalQ's comprehensive Berlin Journal:
http://fodors.com/forums/threadselec...p;tid=34660080

kopp's trip report Prague, Dresden, Berlin, Switz.
http://fodors.com/forums/threadselec...p;tid=34864330

Berlin trip report - caroline_edinburgh
http://www.fodors.com/forums/threads...p;tid=34614269

Michael's Berlin trip report -
http://tinyurl.com/2hmfe6

berlin trip report - michelleNYC
http://www.fodors.com/forums/threads...p;tid=34541911

report of weekend in Berlin - StephCar
http://www.fodors.com/forums/threads...p;tid=34858263

111op's trip report - including dresden (Green Vault) and Berlin:
http://fodors.com/forums/threadselec...p;tid=34882641

my trip report from our first trip:
http://fodors.com/forums/threadselec...p;tid=34981792

I have written a report from the second Berlin trip (in Sept) and should have that posted within a few weeks - have to organize the photos first. It will be far more concise than the first, but I'm sure nowhere near as entertaining as the one that Thingorjus will be posting!

Traveler1959, night walks are just what I had in mind. I love that. I've been living in Manhattan for 7 years but still indulge in night walks regularly. What a way to explore the City and to relax.
By the way, I don't speak German although my boyfriend can manage. But I will try out a quick audio course. Any recommendations?