For most visitors, you'll be eating what you pack in, so freeze-dried meals, candy-nuts-and-granola mixtures, and oatmeal are your best bet for on the trails/lakes. Fires are prohibited except in selected sites with community fire rings or grills, so you'll need a camp stove. Edible berries can be picked and eaten along the trails when in season. Anglers especially with watercraft can add freshly caught fish to the menu. The camp stores at Snug Harbor and Windigo have a limited selection of packaged groceries for housekeeping cabins, freeze-dried food for in the wilderness, and chips/candy for those just returning.
Rock Harbor Lodge Dining Room
Nothing too fancy, but pleasant. This is where lodge guests on the meals-included "American Plan" eat, but "European Plan" guests, cabin guests, and campers are also welcome though if you're just coming in off a week on the trails, buying a shower first would be a nice courtesy. The lake trout is really fresh, but be prepared to pay premium prices for the luxury of kitchen-prepared food in the wilderness.
Greenstone Grill
Simple fare such as burgers, sandwiches, pizza, coffee, and drinks. Very reasonbly priced, given the location, and it's so much nicer to hear the crunch of fresh lettuce and onions on a burger than the crunch of unreconstituted freeze-dried peas in your beef stew.
Windigo camp store
Simple cold sandwiches; the only source of prepared food on the west end of the island.