At FRA, stay airside—use the extensive shopping and lounges, and avoid long terminal transfers unless necessary.
We recommend you stay airside: Frankfurt Airport is efficient but the city isn’t close enough for short exits.
A quick train ride downtown for one walk or meal is possible.
Stay central and keep plans compact. The old town, riverfront, and museums are easy to combine on foot. Public transit is excellent, making it simple to enjoy the city without rushing.