Frankfurt am Main
I’d think twice about going to Munich. It depends on how you’re planning on travelling, but it’s a 3 hour trip by train (one-way) and I don’t know if you would be able to enjoy the Oktoberfest without feeling pressured.
I second the Heidelberg suggestion. It’s very pretty and scenic, but not as exciting as the Oktoberfest… :-)
Have you been to Frankfurt before? If not, you could take the time to explore the city. It has very nice corners that are worth seeing.
I also agree with going to Heidelberg. One of my favorite places – be sure to go up to the castle.
And if you were planning on taking a train to Munich, they’re usually very packed – standing room only – during the Oktoberfest. And if you haven’t booked a hotel room by now, it would be very hard to find one and/or expensive.