Inverhuron Provincial Park our favourite, Pinery has some too. Best Beaches in Ontario on Lake Huron and worlds best sunsets too.
Some site at Rondeau PP aren't bad. You can look at pictures at Ontario Parks website. better book now, the reserved sites fill up quick.
Southern Ontario is a big area depending on who you talk to...the area around Kincardine, Sauble Beach and if you want to go all the way up to the Tobormorary area on the Bruce Pensula. We had some great campsites around the Grange River in Guelph.