We stayed on the Canadian side at the Marriott Fallsview. You can do quite a bit in one day. We went to The Butterfly Conservatory, on the Maid of the Mist, the Journey Behind the Falls and a tour of the power plant all on day one.
I personally would say the main things to do are The Maid of the Mist, and The Butterfly Conservatory (my favorites!). There is a casino, we went twice at night and had fun. Where the casino is is also the area of town that has fun houses, wax museums, arcades etc... There is a Ripley's Believe It Or Not and a Guiness Book of World Records. This area also has: The Hard Rock Cafe, Planet Hollywood and Rain Forrest Cafe. Some wrestling place opened, I think it was a restaurant too.
We went to Marineland, which was ok, or as my husband put it "alot of nothing.". They had a tank with beluga (sp?) whales and one with killer whales. They also have a touching session time with both which was nice. There are rides and animals that you can pet. Again, it was ok but don't expect Sea World. They are building on the grounds an aquarium, so when it is completed I think it will be very nice.
There is alot to do up there, we didn't see everything I would have liked to, but I had promised my husband that this would be an easy relaxed trip, not my usual get out the door but 8:00 and go go go vacation. I guess we'll have to go back LOL!