Buena Vista is a must as it is a historical landmark. They have great wine, great presence/atmosphere and great stories.
Elsewhere in Sonoma...
Carneros District - Artesa (great Pinots), and Domaine Carneros (sparkling wine).
Viansa (VI_cky AN_d SA_m sebastiani)
Sunset Cornerstone Gardens
gundlach bundschu
Benziger and their Imagery Estate winery
Kenwood Estate, Chateau St. Jean and St. Francis.
If you wander more north, then contemplate areas North of Santa Rosa (perhaps a second day).
Take River Road West...
Kendall Jackson
.. then healdsburg...
Gracianna
Dry Creek Vineyards
Bella Vineyards and Caves
If you're up for a third day, let me know.
Sonoma is more laid back and less crowded. slower pace.