If your itinerary allows, you might consider Berlin to Rakotzbrücke to Bad Muskau (UNESCO site shared with Poland) and then to Bautzen (Sorbian region). You could spend the night in Bautzen or the Sächsische Schweiz (e.g. Rathen) or even in Dresden, which is absolutely worth a visit.
We did two nights in Rathen (by the Bastei) a few years ago (after a few nights in Dresden). We could easily have spent far more time exploring the region, but two nights was sufficient to take the ferry over and back in Rathen (worth doing!) and hiking up to the Bastei (you can drive most of the way, but the hike is worth it and not that hard). We then drove to Görlitz for a few hours, then up to Bad Muskau, where we had originally planned to spend two nights but had a reservation mixup and spent one. We did the park in the morning, then hit the Rakotz on the way home.
With a car you can easily hit all of those, but you are missing a lot of cool stuff in the region and selling all of those places very short. With public transport it would be a very tough, long day that would not be fun. If you do want to do it all in one day, I would say a car is a must. Then it's doable--about 6 hours of driving total, giving you the other hours to visit the locations.
The Rakotzbrücke is worth a leisurely hour and a half. The Bastei, assuming you only drive up, is maybe 2-3 if you get the bridge, lunch, and a walk to a view OF the bridge (which you would want to do). So yeah, if it's hit and go because your itinerary doesn't allow for the extra night or many extra stops at interesting places alont the way.