When you arrive in a new town it's hard to avoid the large chain coffee houses and mediocre cafés, so here's a guide to some of the places the locals like. There's nothing fancy about these places, it's more about, "when in Frankfurt, do what the Frankfurters do"
Wacker's (Kornmarkt 9) is top of the list for good coffee served in a way the Germans like it, strong! Wacker's has been roasting coffee since 1914. The shop is basic, with a small seating area indoors and a few tables and chairs outside. At busy periods, when all the seating is taken, locals spill over onto the other side of the street with their coffee cups and sit on the wall to enjoy a few minutes peace and quiet. Wacker's also sell a variety of cakes and individually wrapped chocolates to accompany your coffee.
Stern am Paulsplatz (Neue Krämer 12) is another traditional coffee roaster in Frankfurt. Located close to the Römer and sightseeing in the old town, Stern has plenty of seating both indoors and out. The lounge area upstairs really does feel like someone's front living room! The cakes reportedly come from Konditorei Hollhorst, one of the best bakers in Frankfurt.
Bitter & Zart is also in the old town (Braubachstrasse 14) and offers the air of a 1920's tea and coffee salon. It's famed for it's hot chocolate which is thick, rich and unsweetened (I always laden mine with sugar and order it with whipped cream!) Bitter & Zart offers lunch time snacks as well as home-made cakes, of which one is usually gluten-free. Next door is their chocolate emporium which is also worth a browse.
Naschmarkt am Dom (Domstraße 4) is very close to the cathedral entrance. It's small but offers a good variety of hot and cold drinks and the cakes are baked on the premises. Gluten-free options are also available.
Metropol Cafe am Dom (Weckmarkt 13) is on the south side of the cathedral. This laid back, Bohemian cafe, serves great breakfasts and gigantic pieces of home-made cake. It has a lovely garden which is a sun-trap and a great place to hang out in, with a glass of wine, during the in the summer months.
Tee-haus Ronnefeldt (My Zeil shopping mall, ground floor towards the back entrance) This is the place to go to if you love tea. Ronnefeldt has a long tradition in Frankfurt and has an incredibly large selection of teas to drink on the premises or to buy and take home with you.
Manufactum brot&butter (Bockenheimer Anlage 49-50) is up by the Old Opera House. The cafe is proud of it's artisan foods, freshly baked bread and even offers a glass of unpasteurised milk for those not enjoying a coffee, tea or glass of wine. There is plenty of indoor and outdoor seating and once you've finished snacking you have the chance to visit the Manufactum shop next door which is full of interesting artefacts to browse.