Because you are staying at the Ren, you might consider taking the Lufthansa Airport bus to and from MUC. The Munich North stop is very close to the hotel. If you take the S-bahn, you will need to transfer to the U-Bahn (U6) to the Nordfriedhof station to reach the hotel:
http://www.airportbus-muenchen.de/cm...ome/index.html
http://www.airportbus-muenchen.de/cm...ops/index.html
You can then purchase a lower-cost Tageskarte (day pass) for your inner-city sightseeing needs.