The Jewish Museum is a must if the “installation / sculpture” Fallen Leaves is still there. I’ve never seen any piece of art which comes close to it for the effect it had on me – I’ve just looked at some photos of it and – don’t really know what to say just go.
The Radison Blu Hotel is worth popping into to see the fish tank.
An the Opernpalais on Unter Den Linden has an amazing cake collection.