The tip should be about 10 to 15% in cheaper restaurants or pubs you can get away with rounding up the bill or leaving a few extra coins. Otherwise it's customary to leave at least 20K?-40K? or 1-2. Taxes are always included in the price by law. Many restaurants in heavily-touristed areas (along the river, or with views near the castle) will charge a cover or "kovert" in addition to your meal charge. If this is printed in the menu, you have no recourse. But a restaurant will often add this charge to your bill in a less up-front manner, sometimes after printing in the menu that there is no cover. Anything brought to your table will have a charge associated with it (bread, ketchup, etc.) If you are presented with a hand-scrawled bill at the end of the meal, it is suggested that you take a moment to clarify the charges with your server. This sort of questioning will usually shame the server into removing anything that was incorrectly added. It should be noted that some waiters are impolite especially to people from the eastern part of Europe. Pay no attention to this, and simply find another restaurant.
If you're on the look out for fast food, you won't be able to move without tripping over street vendors serving Czech style hot dogs and mulled wine in the Old Town Square and Wenceslas Square in New Town. If you're after Western-style fast food, the major chains also have a large presence in Wenceslas Square and the area immediately around it. Most beer halls also serve light snacks or meals. Definitely try the hot dogs (párek v rohlíku) they're far superior to the greasy, messy version you get in the West. Small, hollowed out French baguettes are used for the bread, filled with mustard and ketchup, and then the frankfurter is inserted afterwards. This turns the bread into a convenient carry case and means you don't get ketchup all over your hands. Make sure you get mustard, even if you don't normally like it - unfortunately the hot dogs are somewhat flavorless and need that extra bit of kick. Prices range from around 15 crowns for a small one to 45 crowns for the terrifying-looking 'gigant'. Note that size of hot dog relates to girth rather than length. Try the trdelnik, a traditional tube-shaped pastry, which can be found at street vendors in Old Town for 50 crown.