We almost always stay at Holiday Inn on Buffalo Avenue which offers a great view of the Niagara cascades and the Canadian side. It just a couple minutes walk to the falls, a couple minutes walk from Rainbow Mall (really nice) and Prospect Point where you can descend by elevator to the Maid of the mist and the foot of the falls themselves. Very awe inspiring to stand that close and feel the power of the water as is rushes down, never-ending. The illumination in the evening is beautiful.
Goat Island is also very accessible either by foot or car. On Goat Island you can visit Terripan Point which allows you to stand at the brink of the Horseshoe Falls, and also walk to Cave of the winds which allows you to walk to the bottom of the Falls. Goat Island offers a restaurant, snack bars and souvenir shops. The view mobile takes you all around the island allowing you to get on and off at different stops to view the sights.
We find the Niagara Falls Power Project also a worthwhile (free) place to visit. It explains all about how the mighty waters are channeled through to the power plant to in order to generate power for the region.
Niagara Falls Aquarium is nice. Driving or walking, one can cross over the Rainbow Bridge in Niagara Falls Canada to Clifton Hill which offers all kinds of museums.
Skylon Tower which offers a breathtaking view of the area once you arrive at the top ascending in a glass elevator. Here there are shops, revolving restaurant, arcade and many things to do. Make sure the visit included the Table Rock House where you can walk under the Horseshoe Falls. Haven't been there since the Casino in Maple Leaf village opened in December.
The Floral clock is also pretty. So much to do and see and affordable. Hope this helps.