Maid of the Mist (or whatever the Canadian equivalent is now) is essential; don't save
too much! Last time I was there a few years ago, Maid of the Mist (US side) had built a little deck where you could get a Cave of the Winds-like experience of going into the spray on foot without paying any extra. The most important thing is to consider your footwear! You can wear the poncho they give you, but all that water that rolls off your top goes down onto your feet!
If you do go to the American side, there's a nice trail near/between Devil's Hole State Park and Whirlpool State Park. You can hike down right next to the river and see the wild rapids on the Lower Niagara. There are a lot of stairs to access the trail, but it's mild other than that. And it's free! Also in the US, Lewiston is a nice little village north of the Falls with many good places to eat. A family-friendly place there is The Silo: burgers and hot dogs and ice cream right by the river. If you or your kids ever read the book Freedom Crossing, it was set in Lewiston (and it's set there even if you haven't read the book!). Old Fort Niagara is a real gem in the area. It was first built by the French in the early 1700s, then fell to the British, and eventually the Americans; it's been in constant use since then (part of the site is an active duty Coast Guard station today). It's in good shape, because it has been more or less always been taken care of. Since you'll be staying in Canada, though, you may want to check out Fort George. It's right across the river from Old Fort Niagara, and you can get a Discovery Pass to get in for free in 2017 for Canada's 150th birthday. I don't know much about Fort George, though, other than that you can see it from Fort Niagara.
If you don't have this sort of thing near your home, there are many U-pick farms not that far from Niagara Falls. A nice one is Sanger Farms. You get bang for your buck at the local farms.
A little further away, but free or cheap and cool:
- Buffalo's City Hall is a beautiful art deco building and there's a free observation deck
- Eternal Flame at Chestnut Ridge Park. Not even all the locals know about it, but there's a flame fed by natural shale gas deposits behind a waterfall.
- Letchworth State Park - beautiful views, but not exactly adjacent to Niagara Falls. It's not scientific, but it was at the top of USA Today's "Best State Parks" list a year or two ago.
