I'm Canadian so I don't have to go through customs, can't help you there.
Tour the Falls?-- no problem. NOT walking distance (actually it is, but its a hike, especially goign up the hill, towards your hotel!!). The Hilton is 'on a hill' if you will. The Sheraton On The Falls is right at the Falls level, walking distance. But no worries just ask conciege/information they will help you with everything. They usually have a shuttle to the older casino and the Falls (not sure if they charge for that anymore)--takes 5 minutes or so. They have Falls 'tours', just ask information for this.
American cash? always good in Canada. Not sure if they will give you a 'better' rate than your local bank in Philly--probably not actually, may want to exchnage some monies now. Nonetheless, can exchange ANYWHERE in NIagara Falls at their 'exchange rate' (including conveneince stores, the hotel itself, the casino etc).
where your hotel is, 3 'big' hotels lie, ALL near each other: The Marriot, The Hilton (where your staying) and The Sheraton Fallsview Hotel (different than The Sheraton on the Falls). The Hilton has the best location no doubt, and IMO is the best of the 3. You have a Denny's within walking distance as well as a Hooters. The casino across the street has a food court as well--that casino is awesome, better than the older, original one
ASK THE CONCIERGE ANYTHING, they will feed you all the info you need.
have a good time, and welcome to Canada
:toast: