The Karl Marks street area is certainly very beautiful, equally if you are renting a flat/ staying in a hotel there or strolling through it. Nevertheless, buying a simple bottle of water or a snack, or some food home is tricky in that neighbourhood - there is only this little cosy shop available.
In Simply food store you can buy basic foods and drinks, water/coffee, snacks - note down the location as there are no food stores until the Lenin Square in the north or Vasilky the opposite side (where it is more expensive).
There are a lot of nice restaurants around - but just this only food store ;-)