Very pleasant setting on some of the higher land in the valley--different than its neighbors.
How about Bistro Ralphs, Ravenous, or Cyrus.
Almost anywhere right around the plaza! If you've got the money (and a reservation in advance), Cyrus. Also good: Zin, the Charcuterie, Bovolo, and Scopa,
Tasty bread, perfect for taking to picnic.
Zin and Scopa are great for dinner.
For a tighter budget, try a luxury burger at Healdsburg Bar & Grill