Europe is large - let me just give some hints for Germany.

In addition to all the possibilities already mentioned, check the bakeries in Germany. In general the choice of different types of bread, rolls and pastry is amazing. Many bakeries also offer coffee and take-away sandwiches, rolls etc., some even pizza, quiche and other warm stuff. In autumn, try Zwiebelkuchen (onion cake).

Train stations are usually full of opportunities to get something to eat, as are shopping malls.
Larger department stores have self-service restaurants with a wide choice of food.

A German law obliges all restaurants and pubs to put up their menu, or at least part of it, on the outside, so you can check a) what they're having and b) the prices before actually entering. Quite useful.

Keep in mind that German supermarkets are not open 24/7. Max. opening hours are from 9 a.m. (perhaps 8.30) to 8 p.m.Closed on Sundays. Bakeries usually open at 6.30 a.m. but close at 6 p.m.
In airports or main train stations shops may stay open till 10 p.m.

McD is very common and can be found practically everywhere. KFC isn't.