For a typical russian fast food experience and a cheap and good snack go to any Chainaya Lozhka (Чайная Ложка) which is a chain with several outlets in St. Petersburg. For instance on Nevsky Prospect, on Sennaya Ploshchad or on Ligovsky Prospect, just look out for a café with all orange design. They serve Blinys - russian pancakes with salty or sweet fillings, a choice of russian salads and of course tea (the name of the chain means literally tea spoon). It helps if you speak russian though because you have to custom order your blinys.