The museum is a great place to learn about the History of Texas and the west. The exhibits are laid out in mostly chronological order and you can see how Texas and Austin became what they are today. There is also a separate exhibit on ACL so you can learn about that festival.
It's always important to understand the history of where you are going. It tells you a lot about why it is the way it is today. It shows you the character of Austin. The ACL exhibit allows you to see highlights from all the years of the festival without having to have been there.
Check out one of the IMAX movies if you have time. Also, the first Sunday of the month is FREE admission.