As others have said, the codes generally aren't much good during peak season in popular areas. There's no reason for hotels to discount when they have no shortage of customers perfectly willing to pay premium prices.

I don't know if these would work for you but here are a couple of suggestions.

If you want a Marriott-branded property, but your budget is limited, try the Fairfield Inn/Suites in Oakville, ON. Its about an hour away from the falls (non-rush hour). Newer above-average property. Shopping malls and loads of restaurants nearby. Huge theatre complex across the freeway. And...in the opposite direction from the falls...Toronto is only a half hour away.

Closer to the falls... A good budget hotel (but non-Marriott) is the Comfort Inn in St. Catherines, ON. Less than a half hour from the falls. No frills, but clean, comfortable, and a good location at a freeway exit. Several good restaurants within a five minute drive.

Hope this helps!