The next "shopping sunday" in Frankfurt is on november 8th with downtown shops (along the "Zeil") open 1pm till 7pm !

If that is "your day" you´re lucky but expect a busy city then ...

Just take the S-Bahn from airport to the city and step off at "Hauptwache". Journey time ~ 15 mins airport to Hauptwache.

Btw.: There are no restriction for sale of alcoholic drinks besides drinking-age limits (16/18 yrs. depending on the amount of alcohol content (vol.%...)
It´s just that cashiers now much more thoroughly check ID to verify your age compared to some yrs. ago ...
(unless your appearance makes it clear that you´re no more a teenager )
Fines are hefty if they don´t check now !!!