The LBJ National Historic Park is actually divided into two sites - the LBJ Boyhood Home (and attached visitor centre) in Johnson City, TX, and the LBJ Ranch and birthplace site in Stonewall, about 10 miles west of Johnson City on Hwy. 290.
The Johnson City site provides a better introduction to the life and work of LBJ since the Visitor Centre contains a quite well laid out museum and theatre with a film about his life. Visitors can tour his boyhood home a short walk away.
Access to the LBJ ranch is provided only by a NPS tour bus (costs six dollars per person). The tour takes you to LBJ's birthplace (actually a reconstruction of the original), the Johnson family cemetery (in which LBJ is buried), the Texas White House and a barn area to view the LBJ ranch cattles and goats. As the main house is still occupied by the Johnson family, visitors only drive by the outside and see some of the outbuildings constructed when the ranch really did serve as the alternate White House.
The tour stops at two points: for a visit to the birthplace and cemetery and at the barn area to view the animals. I thought the latter stop was a little too lengthy and probably caters more to the younger members of tour groups. Nevertheless, the visit was fascinating and well worth making the trip out of Austin.