Wow, I just about dropped over when I saw SARNIA in the subject line of a thread! I'll make sure to wave out my window as you pass thru my town!
We have 4 McDonalds, all pretty close to exits. The Quinn Drive one posted above is actually the closest, but it is inside a Wal Mart, which makes it perpetually filthy and crowded.....
If you have kids, the one with the playground is actually only about 1 minute past the Quinn Drive one, take the Modeland Road exit - when you come off the exit it splits off in two directions, you take Modeland Road (hwy 40)
south exit, and then at the lights, merge right. McDonalds is your second stoplight, on the right. There is also a nice one off the Front Street exit, one with a fireplace and sofas but no playground, and it is more of a drive thru the city.
If you want to stay and play in Sarnia we have a fantastic public park with sandy beach, huge playground, and fun petting zoo for the kids.....Canatara Park is located just off the bridge, a great place for the kids to run around a bit, or right under the bridge is another wonderful park with huge playground, and a place to feed the fish.....(just a shameless plug for sarnia

) Have a great trip!!