Germans like their Sundays, we do our chores and grocery shopping on Fridays and Saturdays, Friday night and Saturday nights are for going out and Sundays is for family time, staying at home and relaxing in general.
Which means the Streets are empty, most people stay inside and most of the shops are closed.
The schedule for the public transportation will also change.
However, since Frankfurt is a very tourist and expand oriented city there will still be plenty of stuff to do in and around Frankfurt.
You will find that most of the restaurants and Bars are open, and there are lot of options to have brunch and other breakfast offers.
If the wetter is good there will be plenty of opportunities to take a stroll in the park or have a walk on the Main river. You could also take a train outside Frankfurt and visit some of the many old villages, castles or wine yards that surround Frankfurt. But do keep in mind to check in advance what is open and check you train connection.

Regardless the day its always helpful to download the RMV to your phone which lets you check connections in real time and on the fly.
In Frankfurt itself most of the tourist places will be open, as well as the theatres and museums. But here it would also be advisable to check beforehand.
All in all, you won’t be bored on Sundays, but do keep in mind that your options mid be limited somewhat. So, check your trains and the places you want to see beforehand.
And if you have any shopping to do, you should probably pick another day, since you won’t defiantly get the chance on Sundays.
If you need groceries argand there mid be a chance to find a gas station and sometimes you have small local shops open which carry the essentials.
In conclusion, you will probably have a great day if you prepare yourself before.