Birdwood is a pleasant hour's drive from Adelaide via the South Eastern Freeway. Turn off at the Hahndorf/Woodside/Birdwood exit and follow the Onkaparinga Scenic Drive, which takes you through some lovely townships that come alive at different times of the year (such as Oakbank's Easter Racing Carnival and the Lights of Lobethal).
The National Motor Museum is an international centre for the collection, research, preservation, education and display of Australian road transport history. It is open from 10am-5pm each day (except Christmas Day) at a cost of Adult: $12; Concession: $10; Child: $5 (children under 5 free); Family: $30 (2 adults, up to 6 children).
The National Motor Museum houses a whole myriad of cars and motorcycles. I haven't visited there for over 20 years and was seriously impressed – each vehicle was accompanied with details of its history. There are many interactive displays, such as "Life in the 50s" which comes complete with a milk bar, jukebox and drive-in!
Alternatively, take the time to experience the unique Blumberg Hotel; so called because Birdwood was formerly named Blumberg as part of its German origins, until World War I came along and changed all that, resulting in the Anglicised name change to Birdwood (named after Sir William Birdwood, the Australian Imperial Force general who led the ANZACs at Gallipoli.).
How Blumberg became Birdwood (click to enlarge writing)