I would strongly recommend the Oasis--been going there forever and never been disappointed. Buttermilk Cafe is hit and miss though the breakfasts are probably the best bet. I've heard good things about the Northside but still haven't been. They seemed to be closed every time I was interested. Maybe soon.
You have a few good choices:
ButterMilk Cafe (very homestyle for brunch and dinner)
Oasis Bar & Grill (probably most popular place in town)
North Side Grill (a little more upscale)
Cat & Fiddle ( for a pint and very good Pub Food )
these have all withstood the test of time
Daizies is a new brunch place that has quickly become very popular
All these places are in the downtown core, you can walk around and check out the menus. Enjoy!
Oasis Bar & Grill
31 King St E, Cobourg, ON K9A1K6, CA
North Side Grill
92 King St W, Cobourg, ON K9A2M3, CA