I'd recommend the Midland/Penatang area. Just about any spot on Georgian Bay will give you plenty of anchoring spots close at hand. Advantage of Midland/Penatang is the waters are very sheltered, lots of marinas, and good access to the a lot if islands or the main body of Georgian Bay. Disadvatage is that it can get busy at weekend including the more popular anchorages. The narrows into Penatang can look like the 401 on a Sunday afternoon( OK maybe not thaqt bad ).You can go further north as Chantler suggests but that makes for a much longer drive from Toronto and I don't like to spend my weekend driving. As for marinas, I,m at Bay Port in Penatang. Good spot, lots of kids and dogs, plus it has more sailboats than any other marina in the area (about 50/50 ).
Also, if you don't have a chart plotter then get one, and be prepared to do a lot of tacking and gybing at least in the
severn Sound area.