Agreed, it is flatter, but summer heat (and smog alerts) and humidity can make it equally challenging as cycling in the Okanagan Valley, B.C.'s wine country ..for its dry heat and more hills.
If I can recall..from nearly decade ago and earlier, (we moved out to Vancouver in 2002 from Toronto), we did cycle to/in the Niagara region 3 different trips, saw slightly different things:
lst trip --cycling round trip from downtown Toronto to Niagara-on-the-Lake. this was in July, close to 90degrees F, 100% humidity. We camped for 2 nights at a campsite not far from the General Wolfe memorial statute near Lake Ontario. This trip we cycled a little over 300 kms. in total which included diversions to local sites.
last 2 trips-- we used a fast-speed ferry from Toronto, which no longer runs, which dropped us off at Port Dalhousie, which brings one much closer to Niagara-on-the-Lake.
As you might guess, the area is beautiful in late spring or early fall just when the autumn trees turn colour...which covered our last 2 trips.
One of the trips, we stayed at a bed 'n breakfast was run by a painter-artist and her hubby. There are artists' studios which we did drop by a few in the area also.
I don't know how you got to Niagara-on-the-Lake, but we followed the Waterfront Trail from Toronto, which runs through Oakville, Mississauga, with jogs around Burlington, and some more creative planning around towns of Jordan, Grimsby..
I didn't know until I visited there, how steeped the area was in Loyalist history of Canada..which makes a perfect complement to Loyalist history and foil to the Acadian history in Canadian Maritime provinces where we did cycle-tour longer on different trip.
To cycle round trip along that path from Niagara Falls to Fort Erie is 50kms. which is very easy to do.
I miss Ontario and that part of Canada for its beautiful red-orange flaming autumns..and maple syrup! which West coast falls does not replicate much at all. And the blooming lilac trees and bushes at this time in countryside. West coast does not have much lilacs.
Other suggestions is to ride to Royal Botanical Gardens in Hamilton, which would be a full day trip approx. 130 kms. from downtown Toronto. Their lilac tree garden section is probably one of the biggest and most diverse in Canada. I don't think there's anything like that at Butchart Gardens in Victoria, B.C.
Or try a trip eastward, througn Port Perry and onto Coburg. Toronto-Coburg would be 110 kms. one-way. We did it twice. Coburg area is known for apples ...I think near Brighton. If a newbie felt tired, person could catch a VIA train coming from via Kingston to Toronto. Or catch the GO commuter train from Pickering..but make sure you check the GO train schedules for weekends lst.