Theater, New Opera, the first ECB building (Eurotower), Frankfurts green belt, old Opera, skyscrapers, red light district, riverside and Central Station – from Willy-Brandt-Platz one can reach all of these mayor spots in Frankfurt within a 5-minute walk.
I think that makes it a perfect starting point to explore the city. I would recommend taking the green belt and go up north. It will take you directly to the Old Opera with a nice view of some high rises. At the end of this walk you will find the Old Opera building which was rebuilt in the 1970s after being destroyed in 1944. The Opera Square is also a nice hangout in the summertime and from there another spot of mine is just across the street: Check my Brot & Butter article.
By the way old Willy was the chancellor of Germany between 1969-1974 and his “Ostpolitik” is probably the most important legacy; it aimed at improving relations between West Germany and East Germany as well as with the Soviet block.