As one of the most important industrial cities in Germany, Frankfurt suffered more than most from sustained allied bombing during World War II, and as such, many of the city's most historic buildings survive only in pictures. An exception to this is the Romerburg, on the old Market Square, where amongst the surviving treasures are the Romer itself - The Old Town Hall, Paulskirche, and the Frankfurter Dom. Close by, you can also see the museum dedicated to famous German writer Goethe (Faust, The Sorrows of a Young Werther) in the house where he lived for a while.
On the other side of the river, the Museum fur Kommunikation is great fun for kids of all ages who can get hands on with some of the important treasures from the cold war including plenty of invisible ink and other spy gadgets that really get the old James Bond fantasies into overdrive.