You can wander the blocks around the Plaza and you can hardly avoid bumping into a "wine bar" -- many of these places represent wineries, and for a $5 or $10 tasting fee you can sample their wines. Others are true wine bars, where a variety of wines are poured. Some of these are open late.
There are signs on all the corners pointing to the wineries/tasting rooms -- there are at least twenty in town. .
If you stop in at the Chamber of Commerce (217 Healdsburg Avenue) you can get free passes to a number of these tasting rooms.