You need to know what you want to see: Assuming you want to stay downtown you will be in downtown Munich at about 11 am and you will leave for the airport at about 8 am the next day. So not much time "to drive to the castles", in particular when being tired.

Book yourself a room at the Sofitel, very conveniently located to the railway station and easy access to the airport. And then do a bit of research here on this forum: There is plenty of wisdom for one day trips to Munich. This should answer most of your questions and give you plenty of ideas.