Technically just over the Placentia line on the Cal State Fullerton campus, the Fullerton Arboretum is 26 acres of plant collections from around the world: California natives, rare conifers, a desert garden, and a productive citrus grove that nods back to the region's farming past.
Admission is free, hours are generous, and the grounds are walkable in under two hours. The historic Heritage House — a Victorian doctor's residence relocated here — is open for guided visits. It is the most rewarding outdoor stop in this stretch of north Orange County.