Good question. When you specify a time that you did an action, we don't use have+verb to describe the action. So, saying "I visited Niagara Falls last weekend." is the only correct option when using a specific time. You can say "I have visited Niagara Falls." but without the time specifier - more like a response to a question emphasizing a past action.

With the perfect tense you cannot specify the time something occurred. It is only about the fact that it has happened. So you need to use the simple past or delete "last weekend" I love visiting Niagara Falls. I usually go down there two or three times a year