Half a day in Munich - ideas?

I have an 18 hour layover in Munich this weekend (international travel). Arriving at 21:00 on Saturday and leaving at 15:00. I am staying in the Hilton at the airport. Is there anything I can do before I need to be back at the airport? I was originally thinking of going to the BMW museum but that opens only at 10:00 and I need to be back in the airport before 13:00. The travel time is 45 mins so I can get about 2 hours there if I leave the hotel at around 9:00. When do the shops open on Marienplatz on a Sunday? Are they even open? Thanks!