How about somewhere near the Niagara-on-the-lake 'downtown' area? It's cute and romantic, and I know of a few parks around there. I suggest avoiding the floral clock area like the plague, though.
There is a park across from the floral clock where there are a whole bunch of totem poles and a path that leads you to a hiking trail along the whirlpool side of the niagara lake area. there are tables here.
Follow Picton Street going southeast leaving NOTL, it becomes the Queen's Parade and eventually Niagara Parkway, which follows the river flowing from Niagara Falls into Lake Ontario. You can't miss it!