Everything will depend on how long your layover is. Frankfurt International Airport is a very pleasant point of connection, they handle very well all the hundreds of flights that each day go through this truly international terminal, punctuality is almost the norm that is only occasionally affected by very bad winter storms. The airport is very well connected to the downtown using the Metro and there is easy transportation to neighboring cities using the train from the station just minutes away from the arrival area. Here are some suggestions of what it is possible to do while in a long layover. It is good to know that there are lockers for a fee in the airport and in the main train station in the downtown where one can leave luggage to be able to walk around with more freedom.
Stroll around historic Frankfurt: It is very easy to reach the Downtown. It only takes 15 minutes from the airport using the suburban train. Frankfurt is certainly a modern city but there are some nice historic areas full of charming architecture.
Enjoy the river side walk: It is a pleasant way to see the city. The main river along the city offers a wonderful spot to exercise or just to take a walk. Specially in the afternoons and weekends one will find many people along this beautiful park.
Do a little shopping: or maybe a lot, there are many excellent brands that have presence in Frankfurt. Department stores are great places to enjoy an impressive variety of merchandise. Supermarkets boast a amazing selection of fresh products and quality wines. Tourists enjoy duty free shopping when they show their passport and fill out some documents.
Just stay close to the airport and rest a little in one of the good hotels: there are several great properties minutes away from the terminal. Among these are the Hilton, the Garden Inn, the Sheraton and the NH. Most of these hotels offer day rates that allow travelers to use the room from 9 am to 6 pm.
Enjoy the great ambiance of the Squaire: this is an office and entertainment complex built on top of the existing train station close to the Airport. There are some hotels, many good restaurants and excellent shopping opportunities.
Visit one of the amazing Museums: there are many to visit in Frankfurt among these can be highlighted the Stadel Museum that houses one of the most extensive collections of Art in Germany and the Fuer Moderne Kunst that has another impressive collection of art and at the same time it is a great designed building.
Travel to one of the close by towns: Since there is a train station connected to the airport traveling to close by cities is a great idea. Among some beautiful places to see that boast history and architecture are Wiesbaden and Heildelberg.
Just enjoy a good meal in the city: with so many restaurants famous for quality food, dozens of cafes and bakeries just having a gastronomic adventure in Frankfurt may be the perfect idea to relax.