The 28-story Kaiser Center at 300 Lakeside Drive was Oakland's tallest building when it was finished in 1960. According to Wikipedia, it was also the largest office tower west of the Rocky Mountains. Its most unusual feature sits several stories up, on top of the parking garage next door.
A garden on a garage
The Cultural Landscape Foundation (TCLF) says the garden was inspired by the rooftop gardens at Rockefeller Center in New York. It was designed by the landscape architecture firm Osmundson & Staley: Ted Osmundson worked with the client, and associate David Arbegast developed the design. TCLF credits Henry Kaiser with hiring the firm. Wikipedia, citing a National Park Service study, says his son Edgar, who ran Kaiser Industries by then, commissioned the garden after the building had been designed.
TCLF says Oakland granted the company several variances, including permission to fill part of Lake Merritt to enlarge the site, in exchange for a major landscape program. The result is that buildings cover 90 percent of the Kaiser Center's land, and 60 percent of that same area is also landscaped.
What's up there
According to TCLF, the garden opened in 1960 as the first "true" postwar rooftop garden in the United States. It has a large reflecting pool with small fountains, a wooden bridge, rolling lawns, curving paths, benches and 42 mature specimen trees. Much of the hardscape uses aluminum and cement made by Kaiser Industries.
The building today
Wikipedia says BART had its headquarters in the building from 2003 to 2021, and that PG&E planned to buy it and move its headquarters there. Public access to the garden has varied over the years and is not covered by these sources, so confirm before you go. The lake below is covered in our story on the Lake Merritt Wildlife Refuge.