The Niagara Falls area, esp. the Canadian side, has some stellar displays and park land. The old Buffalo parks and parkways, designed by Frederick Law Olmsted in the 1800s, are also worth a tour.

Buffalo and Erie County Botanic Garden
Deleware Park (FL Olmstead design)
Niagara Falls (FL Olmstead design)

Guelph, Ontario - University of Guelph Arboretum (2 hours)
Burlington, Ontario - Royal Botanic Gardens (1 hour)
Niagara Falls, Ontario - Niagara Parks School of Horticulture, Butterfly Conservatory, Greenhouses - 1/2 hour
Toronto, Ontario - lots of different parks
Mississauga, Ontario - Japanese Garden
Stratford, Ontario - gardens around Stratford Festival grounds

Lots and lots of gardens in Rochester, Ithica, Syracuse, Jamestown.

You are a few hours away from Pittsburgh and Cleveland as well.

Check Buffalo paper for garden walks and private garden tours as well.