We enjoy Caribou Munroe's Island ... its a Provincial Campground if you don't mind boondocking (no hook-ups) It's only 20 minutes away from home so very convenient for the weekend getaways. It has a great beach. However, there's a place in Cape Breton called MacLeod's Beach Campground; near Inverness about 10 minutes drive North. It has hookups if needed but also fantastic sites along the beach with a spectacular view with rolling hills and sunsets. (boondock sites) There is a winding brook from the hills that flows through the sandy beach and into the ocean. My kids played there for hours! And its not a crowded place.
Of course if you want to go further North into Cape Breton; the Highlands National Park will not dissappoint. Tonnes of hiking in the Mountains/hills (we call them mountains cause its the biggest hills we have
Swimming at beaches or near water falls. Lots to do if you enjoy the outdoors.