If you plan to go alone, a credit card, or a DEBIT card (the kind that looks and works like a credit card, but you keep the money IN it, rather than paying later) or a prepaid card (works just like the debit) will be necessary. You don't say how old you are, but your family might just like to show you the sights. In exchange situations, that is often the case.
Or you could just go visit my family! They aren't that far away! (just kidding, really...you don't want to meet my old parents!)
Seriously, either put the money into a pay before credit card or an account (if you can in your country) that has a Visa or Mastercard debit card attached to it, OR you can bring the cash, and do it here.
And, no. This category is NOT only for Americans. It is for people who are travelling in America, and frankly, you did a better job of posting the question where it belongs than a lot of Americans do!
Have a great time here!
Oh, and yeah...Niagra Falls is pretty far from Denver, but relatively close, I'm sure, since you must be from someplace like Europe or South America? Might be the only chance you get for years to see it, and it's worth the trip!