for just one day, your best bet would be Killbear Provincial Park near Parry Sound, or Awenda P.P. near Midland. Both are only 2.5 hours drive from Toronto (pending on decent traffic). Don't expect to camp at either of them though; through most of the summer almost all parks in Ontario are already reserved. Even for non-reservable sites, people may line up as early as 9 am. Start checking into it now at
www.ontarioparks.com . It might not be so bad now, as peak camping season is wrapping up. And forget about the long weekend.
You best bet for hiking would be along the
Bruce Trail on the Bruce Peninsula. You could camp at the
Bruce Peninsula National park up there, but it may be full too.
However, the best ways to experience Georgian Bay cannot be done in a day. Georgian Bay has literally thousands upon thousands of islands, and none are easily reached. To get the full effect, it's best to put aside a few days and go canoeing/kayaking. I was just up near Killarney (north shore of G-bay) two days ago for a 5-day canoe trip. It is utter wilderness up there; we canoed 2 hours south from Killarney and had a whole island to ourselves. The water was a gorgeous torquoise green.. you could swim and see down at least 50 feet into the water. An absolute paradise. Camping in the interior of Provincial Parks is easier than car-camping, as the sites are not all reserved. There's Killarney, French River, and the Massassauga, all large wilderness parks. Or, you can camp on Crown Land for free (publically owned land.. call an outfitter to find out where). Of all the Great Lakes, Georgian Bay is by far the most beautifull. Becuase the Canadian Shield graces it's east coast, it feels far more remote than anywhere else.
Another option for experiencing Georgian Bay would be to take the
Island Queen cruise out of Parry Sound. It's a 3-hour cruise through the Thirty-Thousand Islands (largest fresh water archipelago in the world)... it gets you out far enough to see what it's all about.
To sum it up, you'd want to spend a bit more time up there to not be disapointed. The most interesting parts of Georgian Bay are not easily accessed, as major highways do not run adjacent to it. Take the time to explore a few little roads and niches. Get a detailed map; the official Ontario Road map will not help you in the least bit.
Another alternative liesure option you may not have considered would be to scuba dive at Tobermory; it's a popular spot as there are many shipwrecks there. Also, if your up for a party, check out Wasaga Beach for the long weekend. It's the longest freshwater beach in the world (14 km, I beleive) You won't see much natural splendour, but then again, maybe you will