The number one tourist attraction in Germany is Neuschwanstein Castle, you should go there if at all possible.
Only if you have tons of time coming out of your ears. Think Disneyland on steroids for crowds. At some point in the tour I was so saturated with gold, glitz, and royalty I was ready to take an ecape route (they exist). If you must get a dose of King Ludwig II, go for Linderhof. IIRC there are only 6 rooms on display. It's enough to get a sense of Ludwig II's world. One small gem in the area is Wiesenkirche. It's about as fine an example of baroque churches as exists. It's not big, and sits in part on the edge of a large meadow. (wiesen - the church's name translates to "meadow church")
Regensburg is also a nice town and outside of it are several remarkable monuments: Valhalla, the Kaiser Wilhelm memorial, and the Befreiungshalle.
Nuremberg if you are interested in the Nazi background and want to see the stadium that Hitler held the famous rally that you see in old films of that era.
Meh - your pick. FWIW seeing Nazi places is more than a little chilling. Keep in mind, too, that German law prohibits so much as the tiniest swastika hidden somewhere. IMNSHO life is too short to spend on this horror.
I would also go to Munich to have a meal at the Hofbrauhaus at least.
Hofbräuhaus ("hoe-f BROY house) is, in great part, a tourista thing. If you've got time, try it on. The interior is huge. the deal with seating is simple. Grab an open place on a bench. If someone doesn't want the space taken, you'll find out - no hurt, no foul. There are other places, less well known, but have the same feel. Try Augustiner Stammhaus on Neuhauserstrasse (main shopping road in the area). Also worth a stop is the Viktualienmarkt - a huge farmers market.
My particular vice in Bavaria is weisswurst, literally white sausages. They're small and done in hot water. Served right, they'll come in a bowl of hot water, on the plate is killer mustard and whacking big pretzel. Tradition says "no weisswurst after noon". The idea is these puppies are made early in the morning and spoil in the afternoon. That said, almost anywhere will serve them at any time.
There's not so much of interest in Northern Germany and it is flat. Leipzig has the Battle-of-the-Nations monument celebrating one of the bloodiest battles in the Napoleonic Wars. In the northwest is Herman's Denkmal celebrating the famous battle of 9 AD where the Germans completed destroyed 3 Roman legions.
????? Flat, yes, nothing to see. Not the last time I looked (three years ago).
Linking Neuschwanstein and the medieval town of Rothenburg is the Romantic Road.
You probably don't have time for all this, LOL.
Rothenburg ob der Tauber (the Rothenburg on the Tauber River - there's more than one Rothenburg - ditto for Frankfurt BTW)... It's medieval alright. It's walled alright. It's worth a walk-around off season. On season... nah.
There's a low mountain chain the begins basically south of Frankfurt (am Main - on the Main ("mine") River). That includes Odenwald and goes to Schwarzwald (Black Forest). Talk about twisties and ups and downs.
Pick an area and spend time there rather than trying to go from Munich to Berlin to Hamburg to Wherever.