Decent Minnesotan home-cooked, inexpensive food at all of these places. Another Chowhound post recommended Cedarwood in Onamia, we were not impressed, however.
Onamia: Happy's, right on Rte 169. Homemade soups are excellent (wild rice, chicken dumpling). Fresh, good hamburgers, etc., and yummy Indian fry bread tacos on Tuesdays.
For grilling on your campfire or at your cabin: Meat on Mille Lacs, also on 169 in Onamia. Great brats, excellent selection of well-priced meat.
Isle: Going east on 27 turn right at the post office and across from the library is a cute family-run coffee shop that has great homemade soup ( we had taco), panini, wraps, etc., and good coffee.
Brainerd: Going west on 210 before the river (711 Washington St) is the Barn, a small Maid-Rite purveyor that looks like a small log cabin. Maid-Rite is seasoned loose meat on a bun with mustard, pickles and onions, which I did not know until I enjoyed it here. This spot is like stepping 50 years back in time, with great homemade soups (we had bean and split pea) and about 10 homemade pies...the rhubarb custard was gone, of course, but the pecan was fantastic. Almost worth a trip to Brainerd just to try them all--I am sure their sour cream raisin is the Platonic ideal. Super flaky crust on the fruit pies, lemon with meringue piled like a mountain.