Like the nearby Kongresshalle, you can visit here for free without going to the documentation center, but it makes a lot of sense to go to the center first, in order to get a feel for what occurred here during the Nazi era. All that's left are the concrete bleachers and the podium where a number of rallies featuring Hitler took place, as well a ring of concrete pillars around the former extent of the venue. The open areas, which during the times of rallies were filled with party members, Hitler Youth, etc, are now re-purposed as municipal sports fields. It was a bit surreal, as we had seen photos and films of the well-publicized Nazi Party rallies which had taken place there, and then we found ourselves standing in the exact same spot.
The roads around the bleachers can be a little tricky to navigate on foot, particularly if you are going to the S-Bahn station at Frankenstadion, so just keep your bearings when you're coming/going.