We just got back from a 2+ week trip to Boston - Cap Cod - Lake George and Niagara Falls. Niagara Falls was at the end.
Originally, we planned to stay on the Canadian side, however, my DH left the passports at home (making an interesting side trip to the Tip O'Neill Building in Boston, but with the extra fees for expedited cards, we could do a cheap week-end if we needed to go back). However, since I love to use travelzoo, I found a hotel next to the Buffalo galleria mall for $125 per night with a food credit of $100, per night, substantially making it our cheapest ever (as long as you can eat the hotel food - not my favorite, but not too bad).
In Niagara Falls, we purchased the Discovery Pass. It really is only a good deal if you go to everything, but its doable in one or two days. Coming from Florida and having perfect whether there (about 20 degrees cooler than home), we found that we dried quickly after Maid of the Mist.
Cave of the Winds was another, where my kids left off the tops of their ponchos on the Hurricane Deck (incidentially, we kept the ponchos, so if there is a hurricane this year and you see a family in Niagara Falls ponchos in the Bonita area, it is probably us). Blue ponchos for the Maid of the Mist and Yellow for Cave of the Winds.
Trolley is a must and Discovery center and movie is good. The kids liked the Aquarium, but the educator / trainer was probably the least kid friendly I ever saw in such a position. I never saw someone lecture (reprimand) a five year old for asking a question that was part of their presentation. I was even afraid after awhile.
Since we were at burn out level, our last day was shopping for clothes at the Galleria Mall. With teenagers, they were thrilled to get stuff and as mom, its easier to shop with them at a locayion where they know friends will never see them.