Your trip is going to be awesome, and frankly I am jealous.
After 13 trips and nearly 6 months of travel time in Europe, Bavaria is my favorite region, but Budapest is my favorite city.
Oktoberfest is exactly as awesome as you hope it will be.
I don't know how much time you have, but the things near Munich were more interesting to me than Munich itself. (Disclaimer: Munich was great. But the Bavarian Alps region offers far more unique experiences in a relatively small area)
If you've never visited a concentration camp before, you should. Dachau is the most accessible camp to a western Europen tourist destination. If you don't do it here, the chance is likely that you never will. It isn't "fun," but investing one morning of your trip to put life into a little more perspective will make your trip far more memorable than an extra stroll through a few touristy city plazas.
I agree with the poster above that you should carve out some time to visit Berchtesgaden & Hitler's Eagle's Nest. Late September is the perfect time for it to be snowy, icy, & beautiful; but not so snowy & icy that it's closed...at least that was my experience. It really is the most beautiful place in Europe. Afterall, if you were an evil dictator who essentially owned the entire continent, wouldn't you pick the best spot as your own private retreat? I saw & did everything Munich has to offer, and it was great. But I would gladly sacrifice my best day in Munich (other than Oktoberfest itself) to visit the Eagle's Nest.
Neuschwanstein Castle is a must.
I would also suggest another of King Ludwig's castles...Linderhoff Palace. Imagine a palace just as awesome as the Palace of Versailles but with only 10% of the obnoxious stinky tourists.
Garmisch-Partenkirchen is a great way to spend a day in a Bavarian town that looks more like make-believe. There are several similar places to choose from, and I spent a couple of weeks visiting many of them, but this one nestled in the foothills of the Alps is the one that has always lingered in my mind long after the visit.
I'd also recommend a stroll through the Partnach Gorge. It's not well known, but it's a hidden gem that is well worth the time.
Budapest is the most beautiful city in Europe. Some say Prague, but that's probably because they haven't been to Budapest.
I just spent two weeks there, and I bet I ran into less than 20 fellow Americans that whole time. I don't know what you're into: dancing your ass off at the club, high-class dining, local yet fascinating pubs, unique history, fantastic historical museums, beautiful architecture, relaxing spas & mineral baths, chilling by the river, mixing it up with the locals, etc. Budapest really does have it all, and the "overtourism epidemic of Europe" is still a year or two away, so you are hitting it at the right time.
Your trip to Salzburg is probably why I'm most jealous. My visit included 3 days of flooding rains, 35-40 degree weather, & almost total power outage. I would love to visit that city again & likely will since I was robbed last time. Let us know how your trip goes so I can live vicariously.
