Are you a pepper head? I am and its difficult to indulge oneself here in Greece. Mild Mexican salsa is considered HOT here. Of course they don't sell it anyway. Thai or Malaysian food is rare or nonexistent respectively. There are however a few dishes with a little spice in them and one is called spicy cheese or (tiro-kafteri). This is a bread-spread or appetizer dish worth trying. Occasionally you'll run into other spicy treats too. On Mykonos an island specialty is Tiro vol-Yia ! Saltsa Kafteri from the town of Larrissa is reputed to be good hot sauce too! If you want to, ask the waiter if they have anything hot no matter where you are: Kati kaf-te-Ro eparhee?
In Athens check out my Athens Restaurant Guide for locations of Mcdonalds, Kentucky Fried Chicken, Pizza Hut, Taco Bell and a few more. You may run into them on the more touristy bigger islands too but don't count on it. There is TGIF Fridays too if you are desperate for a frozen drink and have a car (its on Kiffisias). The burgers were way overcooked at the Fokionos Negri Applebees IMHO which has since closed. The onion rings were the best thing I had there. I'd go again, I have nothing against corporate mentality they actually called me about it to complain. Like I said I know there is one in Glyfada and a Hagen Daz too.
Goody's & Everest are the two major fast food chains along with a few more like Grigories' & Black Sheep and they are actually better than stateside ones but have different items on the menus. Toasts, sandwiches and assorted pies salty and sweet are the main proffering's along with soft drinks and beer. Why are they better? Well because they have more variety and the food is fresher since it doesn't come by ship frozen from abroad as does, I suspect, all the American fast food sold here. Greece is small so distances traveled to market are shorter and food is fresher. Independent hole in the wall establishments, Mom & Pop's and bakeries all offer food of varying types and many are better than the chains. Bakeries in particular are less expensive, the bread baked that morning and still hot if you get there before 10 am. Sour Dough & Rye bread or sisal is difficult to find unfortunately. My favorite bakery is at 64 Emanuel Benaki in Exarhia and is called ARTOPOIA ZACHAROPLASTIKH. (which I cant even pronounce)
Sweet/Pastry Shops are called Zacharoplastia and there are many to choose from. Some of the more well know are Asimakopoulos, Leonidas and Flocafe. Dodonhi is a good ice cream chain that you will se around a lot too in addition to Hagen Das.
A Galaktopoleon or an Evgha is a dairy store where a simple breakfast may be had and fresh milk, eggs and yogurt obtained.