It really depends on how you organize your trip in my opinion. If you stay in the "Ruhrpott" area you are pretty much guaranteed to pick up a couple of matches/grounds from the area as everything is literally cluttered with clubs (not just talking about Bundesliga, but Buli2 as well or even lower leagues).
Cologne, Gladbach, Leverkusen and Düsseldorf are right around the corner as well, so if you want to get in as many matches as possible then the Northwest of Germany is a good starting point.
I personally would skip Hoffenheim, Augsburg, Mainz, Paderborn, Leipzig, Wolfsburg unless you are a die hard fan and want to get the grounds ticket off.
Every other ground in the league should provide a great match day experience and it might be worth checking out the second division as well. Stuttgart, Nürnberg, Hamburg, St.Pauli are all great locations for a football trip.
I am obviously biased so I would always recommend a trip to Frankfurt, but outside of Frankfurt I think Cologne is great. Proper football ground, great atmosphere, city itself is great as well, easy to get to.
Not sure what the ticket situation is like though as they just got promoted again this year.