We are a married couple (61 / 55) and did our first intervac exchange on Easter this year (Netherlands). It was a very good  experience.
Frankfurt / Main is a city of 750.000 near the Taunus mountains and is located in the middle of Germany with near distances to the romantic river Rhein with the castles and wineyards(0,5h), Heidelberg (1h), Hamburg (4h), München(3,5 h), Berlin (4,5h), Basel / Schweiz (3,5h) or Paris / France(5h) by car.
Frankfurt has many interesting museums and galeries, a nice old centrum (Römerberg) and famous architecture from the medival dome to the housing areas of Ernst May (Barchitecture) to the skyscrapers of today.
Johann-Wolfgang von Goethe was born in Frankfurt, Elias Canetti, Arthur Schopenhauer and Anne Frank (her house is just around the corner from our house). As a high school teacher for fine arts I prepared many informations for intervac visitors.
Our home is calm and near parcs with old trees. The neighbours are friendly and helpful. To the center of the City (Zeil, Konstablerwache) it is about 10 minutes, to the Frankfurt International Airport about 25 minutes by train or car.
We like very much to live in Frankfurt because of the international inhabitants and visitors, the green zones and the cultural events.