Thun has a solid restaurant scene for a city of its size, concentrated in the old town and along the waterfront. Swiss staples — Rösti, raclette, fondue in winter — are available at most traditional restaurants. The Hauptgasse and the Rathausplatz have the highest concentration of cafes and restaurants; quality is generally good and prices are lower than Bern or Zurich.
The covered market at the river end of the old town has a good deli section for picnic supplies if you are heading out on the lake. Several restaurants in the Schadau castle park have terrace seating with lake views — worth the slight premium for the setting on a sunny day.