The one nice thing about Niagara Falls is there are often some American brand and style restaurants that often are in this city because it is close to the USA. Here are a couple that I have visited:
Outback Steakhouse
TGI Friday's
iHOP
Applebee's
Arby
I am not stating that they are not elsewhere (Example: There is an iHOP is located in Brampton, Ontario) but these US brands are more common in this city.
I personally would also recommend you to visit the Flying Saucer Restaurant. This is an American style diner with theme of space and building shaped like a UFO. It is a nice place to enjoy a meal with family and friends. We ordered their Caesar salad, garlic bread, their fried platter and steak and it feels and looks like what you expect from a diner.
Here is a link to their official website: Flying Saucer Restaurant Niagara Falls Family Dining (905) 356-4553
There is also the restaurant in the Skylon Tower that was mentioned on another page: Skylon Tower 360 Restaurant
Besides the above mentioned, there is the typical restaurants and fast food that you would expect from Canada.