Reservations are non-negotiable in ski season. From December through March, every decent restaurant in Ketchum fills up — especially on weekends and around holidays. The Sun Valley Jazz Festival (mid-October), Allen & Company Conference (July), and holiday weeks between Christmas and New Year's are the hardest times to get a table. Book at least a week out for Pioneer Saloon, Ketchum Grill, Michel's Christiania, and Fiamma. During peak weeks, two weeks out is safer.
Ketchum is a walking town. Almost every restaurant in Ketchum sits on or within a block of Main Street. Park once and walk. You can easily hit a cocktail at Warfield, dinner at Sawtooth Club, and dessert somewhere else without moving your car. This also makes it the best town in the valley for bar-hopping — Pioneer Saloon's bar is great for a nightcap.
Hailey is where locals eat. The prices in Hailey run 10-20% lower than Ketchum for comparable quality, and the crowds are thinner. CK's Real Food and TUNDRA are both legitimately excellent — not just "good for Hailey." If you are staying in Ketchum, Hailey is a 15-minute drive south on Highway 75. Worth the trip, especially on busy weekends when Ketchum is slammed.
Check the calendar before you go. Many restaurants in the valley close one night per week — most commonly Monday or Tuesday. Cookbook is closed Sunday and Monday. Others shift seasonally. Always call ahead or check the restaurant's website before making the drive. The Sun Valley Resort restaurants (The Ram, Roundhouse, Trail Creek Cabin) have seasonal schedules and may require resort access during ski season. Trail Creek Cabin is winter-only.