Visiting Frankfurt in summer - need some restaurant recommendations

Hallo,

Me and my girlfriend are visting Frankfurt for The Weeknd concert in the summer. I just got around to planning what to see and what to eat in Frankfurt in the few days we will be there. I'd like to know if there are some traditional restaurants that are worth seeing/eating in.

I'm not very much into fine dining and would rather see and try some local german cuisine and beers. Other than that, we'd like to try german doner too.

We'll stay mainly in the old city center area.