JEFF247, Since you're from N.Y. you are probably already familar with this - but just in case. Remember that you will both need a passport or pasport card (or Nexus cards) to get back into the U.S. Also you should have a letter from your niece's parents (preferably notarized) giving you permission to take her into Canada. The above will save you potentialy
'interesting' hours spent at the border.
This is normaly a very busy time at the falls, but if it isn't too bad, 12 year olds are still the age where they really have an appreciation for such.
Boys more than girls (although plenty of girls as well) also find the Welland Canal of interest.
My niece at that age also found the view of Toronto across the lake from the Brock Monument on Queenston Heights interesting.
(Although as you said, probably better not to tell her about the SPs)
Hope you have a great time.