I've already said a little about Carl von Gontard's Französischer Dom tower from 1785 on Gendarmenmarkt. What I didn't say at the time was that, for 2 euros 50, you can climb the tower (via the stairs, of course - I imagine it'll cost a lot more if you want to do a spiderman) and enjoy the view from the balustrade.
From the ground level, you can look all the way up to the carillon in the dome above, which, with 60 bells, is Berlin's largest. I think the staircase is quite interesting architecturally: the brickwork has a bit of a 19th century industrial kind of feel to it, though it was actually created as part of the 1977 - 1981 restoration of the building.
After you've climbed the 284 steps to the top, you can look all the way down to the decorative tiled floor at the bottom. Of course you're probably not going to want to climb 284 steps just to look at the floor that you saw on the way in. You might want to take a closer look at the bells, of course (and note the one with the inscription "Berlin Stadt des Friedens" - city of peace - a title given to Berlin in 1979 by the World Peace Council), but I imagine that the main attraction for most people is the viewing gallery.
At 40 metres high, it's well above the surrounding rooftops. It's not quite high enough to give you a clear view between the buildings, but still it's an interesting perspective on the city. It's nice to be able to look out without any glass in the way.
I'd actually hoped there'd be some nice warm light over the city like there was the day before, but clouds quickly moved in to shroud the sun, leaving everything looking decidedly cold. To tell the truth, cold is how it felt too. The tower's open until 7 in the evening, so I imagine it would be a good place to watch the city light up after dark, but with temperatures around -9°C and a cold wind blowing, I really didn't fancy waiting up there for that to happen. There's a great view over Gendarmenmarkt though!