IMHO - my Zeil is nice but very hectic and has a lot of young visitors. Start at the „Opera“ (Alte Oper) which is beautiful. From there you have two options - „Fressgass“ and Goethestrasse. In Fressgass there are a number of eating places and some shops. I would personally try Feinkost Meyer (either sit down in full restaurant or buy something over the counter). In Goethestrasse you will find all the luxury brands you know. Still a nice street and good atmosphere. You can also divert to Neue Rothof - even more shops (price point below Goethestrasse, still very good). Then onto Goetheplatz (nice) and then onto „New Oldtown“ where they rebuild a quartier that was destroyed in WW2.

Enjoy - and I would rather take a taxi. Easier than train. Airport is 20 minutes away. Allow for some traffic.

https://frankfurt.de/frankfurt-entde.../neue-altstadt
https://www.meyer-frankfurt.com/delicatessen/fressgass/