For me Frankfurt is one of the most interesting German cities. Certainly during summertime. As for your running, indeed take the walkpath along the river, or the 'green zones' through the city. There is always something to do : either a festival, animation,... in the center, near Romer.
Have fun !
Don't you just love a city that has a river running through it. Frankfurt has the River Main, a tributary of the Rhine river, dissecting it. We spent quite a bit of time walking by the river in Frankfurt, as our hotel was located close to the river and it was an easy way to get to/from the centre of town. I find a river stroll is a wonderful way to take a break from 'traditional' sight-seeing, and relax for a while.
If you prefer to get out onto the water, you can do a river cruise and see some of the sights as you motor by, or even go on a romantic dinner cruise. Boats leave from just near the Eiserner Steg.
Fondest memory: There is a paved path that runs along the city side of the river. I highly recommend a stroll, and there are plenty of benches along the way if you want to take a break and watch the passing scenes.
Favorite thing: Whereever you have a river you will have bridges - and Frankfurt is not any different! I was surprised to find quite a few pedestrian bridges over the river Main - as well as regular street and train bridges as well, of course! For a great variety of pictures, have a look here.
Favorite thing: You can take a cruise along the waterfront and get great views of the city. The speed at which a riverboat travels and the leisurely feel of the ride makes this a great way to get an initial feel for the place. Of course, walking is the best way to truly get to know a place, but the photos are better from a boat!
If you can get your hands on a bike from a friend, hotel or even rental place, this is the way to get around Frankfurt.
Frankfurt is flat and scenic and great to see by bike. Adventure down narrow, cobble-stone streets that you would otherwise not see.
The best bike route is along the River Main. You can ride along the North side of the river for about 35km. Its mostly a beautiful track winding along the river... past small towns, summer beer gardens, old churches...
Go down to the Main river as it is very picturesque. If you get a chance, try to take a trip down the river on one of the tour boats. I would love to tell you what they are like, however, we couldn't find the bloody things! It was one thing that I would have liked to have done.
Fondest memory: One thing I really do miss about Germany as a whole, is the fact that it is so clean.
Frankfurt for a huge city is very clean. They seem to really be into recycling as well with bins everywhere marked off for different paper, plastic and bottle sections.
If only everywhere in the world could be so conscious of how nice a clean city is!
Favorite thing: You can have a nice view of downtown Frankfurt just by walking along the "Main" or on one of its bridges.
Favorite thing: During Summer, stroll, cycle, rollerblade or sit by the River Main and watch the people go by... such a relaxing way to spend a weekend and you really feel the vibe of this small city.
Favorite thing: walk along the river Main. I found it very pleasant to just sit on the grass and watch the boats. The promenade along the river seems like an excellent place to bike, as well.
Favorite thing: Visit the bridge over the Main river. A picturese view of the financial part of the city, so called Frankfurm Manhattan can be seen from there.