Zeilgalerie has a gem of a viewing platform (Dachterrasse) that may be unknown to the general visitor. It is actually on the roof of the building, and it is open from 10am to midnight. Take one of the tiny elevators up, and then climb a few stairs until you are on the outside deck. There are some more metal stairs outside for an even more elevated view (note that these stairs can be slippery in the rain or snow, so take care!). You do get some excellent views of central Frankfurt, including the Dom, the Romerberg, Commerzbank, and other assorted skyscrapers. There is a discreet display pointing out some of the more notable buildings in the "Mainhattan" skyline. Incidentally, there is no fee to the viewing platform, so enjoy the free vistas.

If you are also shopping in the flashy Zeilgalerie, there are a variety of clothing stores, shops, food establishments, as well as an IMAX theater. This seems to be a popular hangout for the younger crowd, a bit edgy and hip for some conservative tastes.