The UC Botanical Garden sits in Strawberry Canyon above the campus, with views across the bay through the trees. Its 34 acres hold more than 20,000 individual plant specimens representing over 12,000 species — one of the most diverse collections in the United States, and genuinely worth an afternoon even for people who don't think of themselves as plant people.
The garden is divided geographically: there are sections for South Africa, Asia, Australia, the Americas, California natives, and Mediterranean flora. The cacti collection is enormous. There is a Japanese pond with breeding newts. Greenhouses hold carnivorous plants, tropical species, and succulents collected from early 20th-century expeditions to China and Tibet.
It is quieter and less visited than you'd expect for a garden of this quality. The hillside setting means you are also, incidentally, in one of the better spots in Berkeley for just sitting outside and looking at the bay.