The Orillia night is on Lake Simcoe, at Mara Provincial Park or McRae Point Provincial Park. Mara's beach is the open-water swim, a short drive south of Orillia, with the loops behind the sand. You walk from the site to the water and back. Car camping and RV pull-throughs share those loops, electrical and non-electrical, and a dump and fill station is there for the morning.
McRae Point faces the same lake near town. The North Campground is a row of large pull-throughs toward the water, fully electrical, and Ontario Parks says those sites accommodate extra large trailers and motorhomes. If you need that extra length, look at RV rentals in Orillia. The South Campground mixes electrical and non-electrical sites.
An afternoon in town still works from either park. Streets in Orillia run down to Lake Couchiching, then you drive back to a Simcoe pad for the night. Mara and McRae Point are the first-night loops, close enough that town and the beach are the same day.
Ontario Parks takes reservations up to five months in advance of arrival at both parks. Book the pad first, then the van. An electrical pull-through is a different overnight from a non-electrical site. On the same lake, Sibbald Point Provincial Park is the Newmarket overnight.
Bass Lake Provincial Park
Bass Lake Provincial Park's sandy beach is the swim, a short drive west of Orillia. RV and car camping spread across four campgrounds around that water, Lakeview, Eastside, Cedar, and Hilltop. People spend the weekend at Bass Lake for that beach and those loops.
Eastside is fully electrical. Lakeview, Cedar, and Hilltop mix electrical and non-electrical sites. If Mara and McRae Point are already booked, this is the overflow west of town.
Trent-Severn Lock 42 at Couchiching
Boats climb or drop at Lock 42 at Couchiching, near Severn Bridge, on the run between Lake Simcoe and Georgian Bay. You watch from the bank as a boat waits in the chamber between the closed gates. Parks Canada calls the station very busy on weekends as boaters make that crossing and return. There is no camping at the lock, so the overnight stays at Mara, McRae Point, or Bass Lake.
The navigation season runs mid-May through mid-October. Outside those months the locks are not operating and the boats are gone. If the day pulls you on toward Georgian Bay, Awenda Provincial Park is the Barrie overnight. Algonquin's Highway 60 corridor is the Toronto trip.