I went to Oktoberfest 2007 and have been back to Munich in 2008 and just last month...
I think that 6 nights in Munich may be one or two too many, personally. (And terribly expensive during Oktoberfest.) Especially when you're only spending 5 nights in Paris/Normandy. I'd take at least one of those nights in Munich and move it to Paris. I spent 4 nights there and it wasn't enough.
However, if you do want to remain in Munich proper for 6 nights, you can make day trips to places like Prien am Chiemsee, Salzburg, Berchtesgaden, Schwangau (with the Neuschwanstein castle), or Fussen.
Rather than staying in Munich as a base and taking trains to each of those locations, you may want to consider renting a car and spending a few nights on the road in the Bavarian countryside.
If you don't already have reservations in Munich for Oktoberfest, you need to make them ASAP. (I actually stayed in Augsburg, about 40 minutes west of Munich by train for half the cost.)
That's my two cents worth... :-) Enjoy your planning!