The Lake Huron shoreline north of Sarnia is the obvious day-trip territory. Grand Bend and Bayfield are the two most popular lakeside towns, an hour to ninety minutes north on Highway 21, both with broad sand beaches and a more polished tourism scene than Sarnia itself. Pinery Provincial Park, between Sarnia and Grand Bend, has the best surviving oak savanna in the province and good camping.
For a US-side detour, Port Huron and the Thumb of Michigan are over the Blue Water Bridge in under fifteen minutes. Detroit is roughly an hour and a half south on the I-94.