Try Old Marina Restaurant on Puslinch Lake, just off Townline Road exit. Lovely waterfront location on a kettle lake. http://www.guelphmercury.com/entertainment/dining/dining_420.html
I agree that it is a very short drive to Cambridge, so if I were you, I would go a little further (out of the way, westbound) and go up Homer Watson Blvd into Kitchener for Turkish lunch at Donair and Kebab House at the corner of Bleams and Homer Watson.
It is not that long a drive to Cambridge, maybe an hour or less. Were you looking for a lunch or snack? Unless you want to go off road, there are really only chains out that way. On the highway, I mean. Enver's in Morristown sounds interesting.
There are a few spots in and around Milton.
If you love food, you'll love the charming Enver's in the little village of Morriston. It's just south of the 401 on Highway 6 South on the west side in the middle of the block. They are not open for lunch early in the week.
Check it out here: