Considering the type of forum this is, are you asking for cheap things to do? There are none.
Unless you stop and get a sandwich and people watch in the park you will be paying some $
Just my opinion, but the Napa area has to be the most overrated shithole in the state. I second the Healdsburg, Russian River, Bodega Bay idea and see some beautiful California Country instead. That' or just drive up into the hills for a picnic
MOD NOTE: Please use details and facts, not hyperbole and insults, to make your point. Thanks!