The holocaust monument is a very big and extraordinary monument that is situated close to Reichstag and Brandenburg Gate. It's definitely worth a visit.
After the wall came down, there was nothing at Potsdamer Platz. Then for years it was a construction site. Nowadays, Potsdamer Platz features the city's skyscraper feeling (Berlin doesn't have many). You can visit the Sony Center, shop in the Potsdamer Platz Arkaden and eat out in one of the many restaurants. There's also a big theatre.