My favorite things to do IN Niagara Falls:
US Side - Whirlpool Jet Boats, Maid of the Mist, Cave of the Winds, Goat Island, Little Sisters Islands
Canadian Side - Hornblower Cruises (formerly Maid of the Mist*), Journey Behind the Falls, Whirlpool Aero Car, White Water Walk, Butterfly Conservatory, Botanical Gardens
*This is the same basic boat ride, just starting and ending on opposite sides of the river.
Check into the passes at NiagaraParks.com and Niagarafallsstatepark.com. You can get a nice discount using them. If you're going to go to both the US and the Canadian side, check both passes out with and without the boat cruise - then get the boat cruise on whichever side makes it a better price.
One thing to really remember is that Niagara Falls is touristy. Don't go there expecting it to be something else. Clifton Hill (on the Canadian side) is very much a carnival midway - go there expecting that and you won't be disappointed.
Also, whatever side of the Falls you visit, be sure to just sit down near the falls and watch the water course over the edge. Feel the spray on your face and just let the experience happen. There was a female travel writer in the late 1800s/early 1900s who visited Niagara Falls - she did all the activities and rushed around, then went home and wrote about what a horrid place it was. Then she decided to try it again and this time she found a quiet spot and just sat and watched the falls. She left with a completely different opinion of the Falls than before.
If you like to read about places you visit, check out T.W. Kriner's Journey to the Brink of Doom and Into the Mad Water (might be two mads). They tell a bunch of stories about people and animals going over the falls, etc. (the story of the german shepherd is wonderful). It makes you think about how much power the Falls have (and what you see during the day is only 3/4 of the full power - 1/2 power at night).
Oh, and don't forget to watch the fireworks display.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)