Great thread!
I have so many more, but will just leave these spots that I try to hit up each year.
Calabogie (Barnet Park) - had a public washroom, picnic tables and overlooks the Calabogie Peaks resort. You could go swimming here if you wanted to.
Barnet Park - Calabogie
Barrys Bay (Crooked Slide Park) - has a clean pit toilet, and a beautiful view of the old logging chute.
Crooked Slide Park - Barry Bay
Algonquin Park (Lake of Two Rivers Beach) - has picnic tables. If no one is around you can ride your bike right up to the water for that amazing photo, but just be careful because the grass can be wet there and you don't want to damage it or get your bike stuck. Awesome views.
Lake of Two Rivers Picnic Ground and Beach · Algonquin Provincial Park
Bancroft (Baptiste Lake) - just a spot beside a big lake. Perfect for a quick break and an awesome view of a lake.
Baptiste Lake - South Baptiste Lake Road
Haliburton (Kennisis Lake) - I love this stop before hitting up Watts Road / Trail, because that road is one of the most silly 1st gear roads in the whole province.
Go slow on Watts Road afterwards
Haliburton (Features Brasserie) - I know you said no restaurants, but I just heard about a place up in Fort Irwin that I want to go check out their Veggie Lasagna. There's a lake there, so I'm hoping to sit by the water and eat it.
Home Features Brasserie restaurant
Minden (Harrington Park) - has picnic tables and portable toilet. Awesome spot to watch some rapids and occasionally I've seen whitewater rescue training occurring there which really neat neat to see.
Minden Whitewater Reserve
Burks Falls (Screaming Heads) - I just love it here.
Screaming Heads · Midlothian
Sprucedale/Huntsville (Buck Lake) - often has a portable toilet and perfect spot to get a photo, go for a swim or just take a break.
Buck Lake Boat Launch
Bracebridge (Huckleberry Rock Lookout) - if you have some time and don't mind a 3km walk, this is one of the best easy hikes to a awesome views of Muskoka. Portable toilet in the parking lot at the trailhead.
Huckleberry Rock Lookout Point - Bracebridge
Windermere Dock (Lake Rosseau) - nearby public washrooms (not always open). Amazing view of the water and perfect spot for a group photo.
Windermere Dock