Frankfurt, and the rest of Germany, no longer has the restrictive laws that once kept stores closed evenings and Saturday afternoons, the very times working people might want to shop. Stores now can stay open until 10 pm, but pretty much everything is still closed on Sunday except for restaurants and bakeries.
The tree-shaded pedestrian zone of the Zeil is said to be one of the richest shopping strips in Germany. The section between Hauptwache and Konstablerwache is famous for its incredible variety of department and specialty stores. But there's much more to downtown shopping. The subway station below the Hauptwache also doubles as a vast underground mall. West of the Hauptwache are two parallel streets highly regarded by shoppers. One is the luxurious Goethestrasse, lined with trendy boutiques, art galleries, jewelry stores, and antiques shops. The other is Grosse Bockenheimer Strasse, better known as the Fressgass ("Pig-Out Alley"), an extension of the Zeil that's lined with cafés, restaurants, and pricey food shops.
One gift that's typical of the city is the Apfelwein. You can get a bottle of it at any grocery store, but more enduring souvenirs would be the Bembel pottery pitchers and ribbed glasses that are an equal part of the Apfelwein tradition. Then there is the sausage. You can get the "original hot dog" in cans or vacuum-packed at any grocery store.