The Fitness Center at Niagara Falls Marriott on the Falls
The fitness center at Niagara Falls Marriott on the Falls is recommended by those who travel for a living. Out of 3 publications that we tracked, 2 referenced the fitness center, such as Oyster and Star Service.
Niagara Falls Marriott on the Falls 
Oyster
Star Service
Oyster "The upper-middle-range Marriott Gateway on the Falls is a tall chain hotel with a quiet location, close proximity to Niagara Falls and nearby attractions, and plenty of on-site amenities (including an indoor pool and fitness center)"
Star Service "The well-equipped gym is down in the basement, and the third floor provides access to an observation deck"
