If you are a fan of The Sound of Music you can check out the garden where they filmed part of Do-Re-Mi being sung for free. You can watch the clip HERE of the movie where the are at Mirabell Palace & Gardens from 1:00 minute to 1:45. If you are not a fan I suggest checking it out simply for the gorgeous castle views. The palace itself dates back to 1606.
Gretly Braun, Adolf Hitler's sister in law married an SS officer here as well. You can read more about the history of the gardens and the palace HERE. I suggest going in the morning because by 11 am the gardens were crowded already with tourists on this warm September morning.