Complete speculation: outskirts of college towns, some of the inland mountain towns in northern California like Quincy. The southern ones (Big Bear, Idyllwild etc.) are already swarmed. Even with these towns, you'd probably have to find something in a less desirable area and wait a decade or two for it to be gentrified. Many hipster or super expensive places were once the "bad area" of a big city: Venice Beach, warehouse district of Los Angeles, Pioneer Square in Seattle. Full of bums, needles on the ground. Super cheap.
The older people I met who lucked out with massive appreciation on their properties just bought houses/condos in cheaper areas with decent locations (commuting distance to the city center), then lucked out when gentrification passed over. At the time they were very bland areas.