It seems like they'd have amazing sex clubs in Berlin.

Kathe Kollwitz museum. Wonderful breakfast cafe next door.
Tour the Reichstag.
Tour the Wall. We found the section near Prenzlauerberg compelling.
Berlin's museums are world class, for art and history.
I think the best part, though, is in the neighborhoods. Central Berlin was bombed out (Alex-platz) and the subsequent rebuilding is not attractive.
Minimize time around Checkpoint Charlie. Too much of a zoo and lots of heckling.

I love Berlin. Not sure what time of year you'll be there, but there's a huge park (Tiergarten) where you can rent boats and paddle around, and near the place where you rent them is a wonderful, huge beer garden.

The "Jewish museum" (that's its name) is huuuuuuuge and well-done. It starts at the beginning, so I learned a lot about Jewish populations in Europe well before the Holocaust.

On that same note, it's not hard to get to a nearby concentration camp Sachsenhausen, which is worth seeing if you've never been to one.

Less depressingly, people speak fluent English, German is easy to muddle through, the afternoon snack is a slice of cake with coffee (can't go wrong with that), and the food otherwise is fun.

There's a political prison there as a tourist attraction. Tours are given by former detainees. It was awesome.

A book that made me want to visit Berlin is called, "My Berlin Kitchen" by Luisa Weiss. It is a "foodie" book but she made Berlin sound so appealing. You might enjoy it before your trip.

I would recommend Potsdam. Historic and interesting. Not glitzy.

Thanks for the replies so far.
We(me and gf) will only be in Berlin for the week. The Potsdam palaces sounds great. As does the museums.

In many ways Berlin is not the easiest city for a tourist. It is not Munich at all (if you know Munich) or a typical European city with a compact and historic and lovely city center that's easy to get around.

Berlin is a large, sprawling city. Because of the history of the war (near total destruction of the city center and the split into East/West Berlin for 45 years afterwards), Berlin has a disjointed feel to it. The old center, Mitte with the museum island, Brandenberg Gate, Unter den Linden and the other WWII/Cold War museums have been rebuilt but has a distinct touristy feel to it and you easily can tell it's not the true center of the city for Berliners, who primarily live scattered across the various districts around the center and hop from district to district and spend little time in the center proper. Because the sights are scattered it can also take a while to get around, although the public transportation system is top notch with just about everything within easy walking distance of a s-bahn/u-bahn stop. The urban feel is also fragmented, you have surviving pre-WWII neighborhoods cheek and jowl with 1950s Soviet tower block apartments and on the whole it's not a pretty city.

Things to do:

The museums are excellent. The main ones are on Museum Island and do not miss the gallery that houses the bust of Nefertiti, which is one of the few genuine masterpieces in the world. It's breathtakingly beautiful. There is another cluster of museums over by Potzdamer Platz (another sort of failed attempt at recreating a pre-WWII landmark area), which get far less visitor but the art gallery has excellent old masters. There are also museums about WWII and the Cold War, all in Mitte.

For palaces, go to Schloss Charlottenberg (largely reconstructed but still lovely), or go out to Potsdam for the royal palaces. Well worth a visit.

Shopping: Kurfürstendamm is the major shopping street in West Berlin. KaDeWe is a fabulous department store with a wonderful food hall on the top floor. Great for lunches. Note: do NOT go to the buffet style restaurant on the very top floor, but eat at the food stands around the food hall.

There's a lot of great restaurants all over Berlin. I hesitate to spend time offering suggestions because what people want from dining varies greatly so if you want restaurant advice, tell me what kind of restaurant you like and what kind of food you want.

I do not know if it is pertinent to you but Berlin has one of the biggest LGBT scene in Europe and a very well established "gayborhood" with plenty of bars and restaurants catering to LGBT people of all ages.

I think that Berlin is amazing this year. I used to travel across Europ from Ukraine by car ( I just rented one at http://ukr-prokat.com/en/ ) and was in Berlin and Dresden as well. Germany is really pretty and colorful country/ What else are you going to visit?

Great city for a bike tour. Flat, huge, and so much to learn.

I am going to Berlin in a couple weeks and would like to hear what others have enjoyed there.
I have a list from some friends, but let me hear what you liked, please.
FWIW, my traveling companion and I are in our 50's and quite mobile.