Petaluma's Lagunitas Tap Room is a great place to spend an afternoon. They offer great beer, food, and live music. There is a grassy area where you can hang out in the sun.
Downtown Petaluma is a great place to walk around and go shopping (it is the antique capital of the US) , eat yummy food, and drink yummy drinks and craft beers.
Food
Sugo Trattoria - casual Italian restaurant that marries the traditions of the old world with the food culture of Sonoma County. http://www.sugotrattoria.com/
Hallies - Great breakfast. Try the infamous cornmeal pancakes. https://www.yelp.com/biz/hallies-diner-petaluma
Sax's Joint - Old fashioned diner with the biggest biscuits you've ever seen. http://www.saxsjoint.com/
Central Market - For an unforgettable lunch, start your meal with the warm olives and finish with the raved-about butterscotch pot o' cream. The restaurant is a top choice for vino fans, as they always stock a great selection of local wines. http://www.centralmarketpetaluma.com/
Wild Goat Bistro - This restaurant that sits on the Petaluma River, and the atmosphere is rustic and inviting. http://www.wildgoatbistro.com/
Food Bar - casual modern cafe using the freshest organic greens and local purveyors. http://www.foodbarpetaluma.com/
Mi Pueblo - Delicious Mexican food. https://www.mipueblorest.com/
Old Chicago Pizza - Located on the second floor, this place has great views of downtown, plus great Chicago style pizza. http://oldchgo.com/
Bars
And so many more!