Love this route!
I began at a winery parking lot (just south of Niagara-on-the-Lake) who let me keep my car there overnight (more on that below). The entire circle is 87 miles (140 km).
The "route" consists of four local trails merged together to make the GNCR:
--Niagara River Recreation Trail (Fort Erie-Niagara Falls-NOTL)
--Waterfront Trail (NOTL-Port Weller)
--Welland Canals Parkway Trail (Port Weller-Welland-Port Colborne)
--Friendship Trail (Port Colborne-Fort Erie)
I took two days to complete it with an overnight stop in Port Colborne but others may be able to knock it out in one day.
Dozens of wineries, the Niagara Gorge, Queenston Heights, N-O-T-L, old forts/history, fruit markets, an active shipping canal (I did catch a Great Lakes freighter, super impressive), and of course, the Falls themselves. 90% paved trail with the only non separated part being the road that is part of the Waterfront Trail between NOTL and Port Weller. Really flat except the lone climb up the Niagara Escarpment but it wasnt that bad and over quickly.
There a tiny bit of road riding near the tourist areas of Niagara Falls especially as you head north right at the rainbow bridge, but then its all path to NOTL. As others have mentioned, the immediate falls viewing area will force you to dismount due to tons of people.
On the Friendship Trail as you approach Fort Erie and the Buffalo skyline, there are many public access points to the beaches along Lake Erie if you want to soak your feet or take a dip, it's quite lovely.
Bring some sunscreen. The Welland Canal section has few trees. My only criticism of the trail would be the lack of signs going through the city of Welland where the trail zig-zags at street crossings and I recall trying to get my bearings to figure out where I had to go. That was 5 years ago.
Here's the "Park 'n' Pedal" Program link if your driving in and need somewhere to leave your vehicle overnight:
https://www.niagaracyclingtourism.com/news/niagara-on-the-lake-park-and-pedal-program/John